Well...the best way would be to probably cut a tiny bit big and then fine tune your cut.Better to cut too big than too small.You should really get a table saw or have someone else cut it though,it's kind of hard to get a straight cut with a circular saw but it can be done.
2 C-clamps and a straight edge....perfect cuts. Set up the straight edge kinda like the fence on a table saw...clamp it down, and follow the straight edge w/ the circular saw.
