Inside Spread is the the longest length you can get at any two points(on the inside of the beams) on both beams. It does not have to be straight across. Just move your tape measure around until you have the greatest length possible. I bet that last deer you got has a bigtime spread. Measure it up and let me know. That was a great buck.

Here is a great thread on how to score a buck...it has helped me a lot:

http://www.huntingnet.com/forum/tm.aspx?m=2529213&mpage=1