I like to use a drafting compass.
http://uploads.tapatalk-cdn.com/2016...92e64de840.jpg
Put one end in the pivot hole in the blade, open it till the other end reaches the end of the blade. Now you have the exact length of the blade. Then put the needle end in the pivot hole of the scales, and mark the wedge end of the scales. Hope this helps you.
A little tip from geezer, that works flawlessly for me.