http://www.rb-designs.com/
Go to the head mods page then the squish band test page, everything you need to know on how to test.
The suggested correct squish as per the book "two stroke performance tuning" by a graham bell (the bible of 2t tuning) is 1.25 mm for two fifty, 1.3 mm for 300 plus or minus machining tolerance.
Cheers
PS I'm not a tuner but read a lot.
