As my son no spends more time reading car magazines then legos, I have acquired some of his legos for use. Recenlty I used some to make a watereline marking jig for my Hermione La Fayette build. Used some wire to hold the pencil tight, the nubs in the legos held the pencil nice and square.
Yes it is a very good and simple idea
I do not know how thick the Lego-blocks are, I guess the smallest appr. 5mm - means with this you can produce levels in steps of 5mm correctness.
=> if more accuracy is necessary you could produce a simple wooden block and cut it to the correct height