You want enough initial to build enough cyl psi for it to want to fire off immed and 10-15 is a good figure for a stock 70's low comp SB but for a measured 10-1 w I'm assuming a hotter cam along w it then 15 would b a good ballpark and this is just to get it started/broke in & you'll set the total at 35 (vac plugged) right when it fires. Later after breakin you can get the initial spot on w the vac gauge method then w that figure in hand shorten the slots to get 35 total (vac adv disconected) then play w the springs then hookup/adj the vac can. I'm assuming you have enough octane to support the 10-1. Dont need detonation issues to interfere w your breakin