When I am "squaring up" a rectangle foundation etc...rather than pulling the tape diagionally until the measurment is the same, is there a formula that will tell you the correct diagonal measurement. My dimensions are 30' x 20'. Thanks

There is the 3 4 5 rule to determine a right angle. Start in a corner, go one way and make a mark at 3'. Then go the other way and mark at 4'. The diagonal measurement between the marks should be 5'

For larger jobs like yours, multiply the 3 4 5 by a number - for example, 5. So 3 4 5 becomes 15 20 25