Its value property is a type of integer. I could not find what its range is, but it seems it is 0 to 100. So, if I set a value like 50, it displays a half-full progress bar. But what if I want to display fractional percentages like "99.9"? Does CellRendererProgress have something like fractional mode? Or is there a different renderer that can take fractional values?
I did a quick review of the "CellRendererProgress" widget by reviewing the GTK source code for the widget and it indeed expects an integer value from zero to one hundred. It does not allow for a fraction as does a regular progress bar. Technically, the range for a progress bar widget is from zero to one, but the "fraction" element is a "double" so a fraction of ".999" (99.9%) would work with it. If you are keen on creating your own custom cell renderer that contains a progress bar in lieu of using the "CellRenderProgress" widget, I did a bit of searching on the web and found this link.
"https://blogs.igalia.com/csaavedra/documents/treeview-tutorial/sec-custom-cell-renderers.html"
You might be able to base a custom cell renderer widget with an embedded progress bar on this example. Bear in mind, this code was written for GTK2, so some tweaking would need to be done.
Regards.