How to measure radius of rounded corner-without scripts

Forgive me if this is a duplicate post.  (I can't find the one I thought I posted earlier today.)  I am drafting a pattern for a messenger bag.  I know I can duplicate rounded corners from one pattern piece to another by copying etcI, but it made me wonder how to actually measure the radius of a rounded corner.  I found some posts by others using scripts.  But I dont' know how to use scripts.  

I have attached my file to show how I approximated it by measuring from the corner of a square of similar size (to the rounded corner) to the radius of a larger rectangle on top of it.  (See the Flap piece - the left hand rectangle)

Should I just bite the bullet and learn how to script?  Or does someone have a clever way of doing it without scripting.  Linked File