UI.Text font size is somehow resized >_<

I'm really sorry, if i'm asking something obvious / already answered stuff

I'm using: "Scale with screen size/Match width or height" Canvas Constant size Panel Constant size Text element with fixed font size and "best fit" OFF

Now, what I really need - is to access rectangles of separate characters of that Text. But I couldn't find any good solution on that, so I decided to cheat a little. I used monospace font, set a bunch of constants to calculate (with some precision) those rectangles. And it was fine, until I tried to run the same project at different resolution monitor. Somehow the actual font size is changed, although Editor values look identically. But I can see that while there was all 30 characters of some fixed string fitting in "Width:500" Text element before, now it's only 29 (because all characters got scaled a little).

Why is it happening and how can it be fixed?