The reality is, it's a guessing game and it's never going to match exactly.
I lost count of how many times I heard "that text doesn't match the image" and had to explain that live HTML text is a different animal than text in a raster image.
A great deal of this issue specifically is why building full page Photoshop mock-ups was a workflow I feel died in ...
As of Apr 4th 2020, I just found an easier offline solution for this issue, especially if you have the shape already in PowerPoint.
Just copy the shape(s) to a new PowerPoint presentation and save it as a .ppt file.
Upload it to Drive. The uploaded file will open in Slides and the shape(s) will maintain their vector properties.
Copy the shapes from the new ...