This is a great AppleT.
Like:
the most complete vernier caliper.
( metric n inch ) like a real one.
depth measurement

well done!

You can and should include  this into your list for checking out too. :)
http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=684.0
Ejs open source Vernier calipers java applet with objects, help & 0-error logic

there are other good features to include.
Like:
your input field that you got me to understand how to make.