One last suggestion to add: Always make the more pieced block sections first. In this case it would be the 4-patch section.
After you've made those, measure to be sure you've got the size required; if they aren't correct, look at your seam allowance. If you need to make corrections now, it'll only be in part of the block rather than the whole block.
THEN make your half square triangle sections using the 1" larger suggestion from MTS. These you can easily trim down.
By the way, I agree, rely on your ruler for measuring rather than your cutting mat, no matter what brand!
Jan in VA