for scantrons thats about the only way u can cheat, but id say about a 30-40% hue would do quite nicely, i tried it out once. getting the right shadeing is TRICKY and requires practice and trial and error. i got to be a TA for a while and actually graded scantrons during HS. last note, the master bubble doesnt work, since u scan the master scantron first and the answers wont reset if you introduce another master key. the only way to get it to work is by reseting the machine and making ursa the key which is highly unlikely : /