Sounds like you did the right thing. If you left yourself 1/2" to trim then it's 1/4 from first two sides and 1/4 from other two sides. When you lay your block on the mat to trim, lay it on point from yourself. Put your 1/1 point of the ruler at 3 o'clock, lining up diagonals like you did, and cut two sides before you move the ruler. Then turn the block end for end and trim the other two sides. That way you're not getting confused about which side you've already trimmed. Hope this helps.