First time timing with new TQ, help

New restified 800 TQ, 340, 4 spd, electronic ign. Put a timing light on it at idle and it reads 10 btdc. Quite often on this thread it is said to be at 35 advanced at about 2500 rpm. When I rev it the mark disappears, so how do you know its 35? Stop laughing, it has been over 30 years since i've done this=P~