If you're not racing the car looking for every last tenth, here is how I do it. Warmed up idling. Place your hand firmly on the valve cover. Get a feel for the engine's vibration before you change the timing. Now, loosen the distrubutor JUST enough that you can move it. Now, pull the distrubutor counter clockwise (advanced) while holding your hand firmly on the valve cover. When you advance too far, the engine will pronounce a very distinctive "rumble" type vibration through the valve cover that was not there to begin with. You cannot miss it. It will slap you upside the head when you get there. Now, back off until that deep rumble vibration just does stop, plus a little bit more. Lock the distributor down and drive it. If it spark knocks any, or it's hard to start hot, come back and keep rotating it clockwise in small increments until it does not. Chances are, if you do it just like that, you'll not have to adjust it again. The key is to make sure you get a good feel through the valve cover of how the engine feels running BEFORE you make a change.
