The 'only' time I would use my cutting mat to measure with is if I needed a large block and if it is off a degree here or there it wouldn't matter because that isn't the finished size of the block. I don't use the matt for the same reason I was told to use the same ruler thru out a project; they can be off sometimes and if you are doing small pieces size can make a big difference in the block. I'm not willing to take a chance of it not turning out ok because I measured w/the wrong thing.