formular to calculate width/height (relative to parent) of container with translateZ inside of parent container with perspective

Question

What is the formular to calculate the widths/heights of child elements with translateZ inside of parent container with set perspective (keyword: "parallax") relative to its parents width/height?

I'd like to create a site with parallax effect on both axis. I was able to figure out everything i need for my mockup except one thing. How to calculate the childrens widths/heights when its above 100%. Because of parents perspective and childrens translateZ the childrens widths/heights visually don't align with parents width/height anymore.

The formular to scale the child elements is: 1 + (translateZ * -1) / perspective. But i was not able to find a formular for width/height. BTW: When childrens widths/heights <= 100% everything works fine.
But see the result on the image below when width >= 100% (containers have top offset to make things visible).

To be correct the approach in my particular case is to let all child elements have visually the same widths/heights.

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in SASS (preferred): PEN or SassMeister
in CSS: PEN

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links from the specs that could help:
https://www.w3.org/TR/css-transforms-1/#recomposing-to-a-3d-matrix
https://www.w3.org/TR/css-transforms-1/#mathematical-description

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"Googled" a lot but didn't find anything pointing me to the right direction. Thanks in advance...

html, body {
  height: 100%;
  overflow: hidden;
  width: 100%;
}

#projection {
  perspective: 1px;
  perspective-origin: 0 0;
  height: 100%;
  overflow: auto;
  width: 100%;
}

.pro {
  transform: scale(1) translate(0px, 0px) translateZ(0px);
  height: 100%;
  position: absolute;
  transform-origin: 0 0;
  transform-style: preserve-3d;
  width: 100%;
}

.pro--1 {
  transform: scale(4) translate(0px, 0px) translateZ(-3px);
  width: 110%;
}

.pro--2 {
  transform: scale(3) translate(0px, 50%) translateZ(-2px);
  width: 110%;
}

.pro--3 {
  transform: scale(2) translate(0px, 100%) translateZ(-1px);
  width: 110%;
}

.pro {
  background: #333;
  box-shadow: inset 0 0 0 5px orange;
  color: orange;
  font-size: 4em;
  line-height: 1em;
  text-align: center;
}

.pro--2 {
  background: rgba(75, 75, 75, 0.5);
  box-shadow: inset 0 0 0 5px green;
  color: green;
  line-height: 4em;
}

.pro--3 {
  background: rgba(75, 75, 75, 0.5);
  box-shadow: inset 0 0 0 5px white;
  color: white;
  line-height: 7em;
}

<div id="projection">
  <div class="pro pro--1">pro--1</div>
  <div class="pro pro--2">pro--2</div>
  <div class="pro pro--3">pro--3</div>
</div>

SASS

@mixin  projection($translateZ: 0, $translateX: 0, $translateY: 0, $width: 0, $height: 0, $perspective: $perspective)

  // strip and sanitize units for further calculations
  // units must be "px" for both $translateZ and $perspective
  $unit: unit( $translateZ )
  @if '' != $unit
    $translateZ: $translateZ / ($translateZ * 0 + 1)
    @if 'px' != $unit
      @warn '$translateZ must have "px" as unit!'

  $unit: unit( $perspective )
  @if '' != $unit
    $perspective: $perspective / ($perspective * 0 + 1)
    @if 'px' != $unit
      @warn '$perspective must have "px" as unit!'

  $unit: 0px // yeah - technically this is no unit
  
  // calculate scaling factor
  $scale: 1 + ($translateZ * -1) / $perspective

  // sanitize units for translateX, translateY, translateZ
  $translateZ: $translateZ + $unit
  @if unitless( $translateX )
    $translateX: $translateX + $unit
  @if unitless( $translateY )
    $translateY: $translateY + $unit

  // render css "transform: scale() translate(x, y) translateZ()"
  transform: scale( $scale ) translate($translateX, $translateY) translateZ( $translateZ + $unit )

$width: 110% // 100% works like a charme
$translateZ--1: -3 // "px" will be added in mixin
$translateZ--2: -2
$translateZ--3: -1
$perspective: 1

html, body
  height: 100%
  overflow: hidden
  width: 100%

#projection
  perspective: $perspective + 0px
  perspective-origin: 0 0
  height: 100%
  overflow: auto
  width: 100%

.pro
  @include projection()
  height: 100%
  position: absolute
  transform-origin: 0 0
  transform-style: preserve-3d
  width: 100%

.pro--1
  @include projection( $translateZ--1 )
  width: $width

.pro--2
  @include projection( $translateZ--2, 0, 50% )
  width: $width

.pro--3
  @include projection( $translateZ--3, 0, 100% )
  width: $width

Answer 1

The formular to calculate the widths/heights of child elements with translateZ inside of parent container with set perspective is:
$scale = 1 + (translateZ * -1) / perspective.

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Where to throw this formular at transform: scale( $scale ).
perspective is parents perspective value and translateZ is elements translateZ value as well. See snippet at the bottom or SASS mockup as you like.

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So actually the answer was given in the question!
While this is the correct answer to the asked question this didn't solve my specific problem at all so here is a follow-up-question.

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Credits: @Trolleymusic who at least opened my eyes somehow with his answer.

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CSS example:

/* formular: 1 + (translateZ * -1) / perspective */

#parent {
  perspective: 10px;
}

.child--1 { /* 1 + (-100 * -1) / 10 */
  transform: translateZ(-100px) scale( 11 );
}

.child--2 { /* 1 + (-200 * -1) / 10 */
  transform: translateZ(-200px) scale( 21 );
}

.child--3 { /* 1 + (1 * -1) / 10 */
  transform: translateZ(1px) scale( 0.9 );
}