Will this work for you? I did a Google search for "quilt math".
http://www.thecozyquiltpatch.com/LCScookbook/
For HSTs (half-square triangles), add 7/8" to the size of the finished block. You want yours to finish at 2 1/2". So you would cut your squares at 3 3/8" to start.
p.s. that page might be a good one to print out - it has lots of good info!