Correct width and Em size


I've been designing my first typeface for the last few weeks and I run into a question:

How do you know the proper width for each glyph?

I'm worrying that, for instance, the width of my "A" is too long compared to other letters. Is there a mathematical way or something like that, other than just the naked eye to figure out the correct proportions?

I'm mainly having this issue with the Caps.

Thanks in advance!