Call me crazy, but this is one of reasons I loved piecing quilts! Color, creativity, and the challenge of figuring it all out! And yes, I am a believer that math is beautiful! (If you doubt it, check out tessellations, fractals, et al.)
PaperPrincess is correct in reducing the starting lengths of sides A and B of the triangle by 1/2" to 4 3/8". (I was wrong in reducing them by only 1/4".) Her calculation of 6.18" (~6") looks right to me as the finished size of the block. (While I enjoyed reading it, I don't agree with the next step she took, however, which reduced the block to 5".)
I am thoroughly enjoying the other ways that people are approaching this question! Vive les differences! (One proof of math's beauty is that it works no matter which way one comes at it.)
eeraemore, PLEASE let us know what you learn from experience!!!
Thanks!