Don't know if this is the type of answer you are looking for, but this is always where I start. For a square or rectangular quilt, if you lay the quilt out on the floor and measure the diagonals, the measurements should be the same. If I am way off in the diagonals, I then accurately measure all 4 sides (op sides should be equal). Then trim as needed (I make my border a wee bit larger than I nee just in case I need to do this)..