Point X is when the advance starts and point Y is when the advance is maxed out. Single/double means (1) spring or (2) springs of the OE light springs that come (1) to a dist. Reportedly the slot length/adv amount chart ain't dead on so file em slightly less than the chart because if you go too far it means another trip to the welder & yes you will have to be in & out of there several times till you get it dead on. I would find the initial your eng wants with the "vac gauge method" then shorten the slots to get 35 (SB) or 36-38(BB) total (both initial and total set with vac adv capped) then work with the springs then plug in/adj the vac adv. EDIT I see that race unit has no vac adv so that's 1 less subsystem to tune. After setting initial and total I'd mix/match springs till you get just under the pinging point at WOT up thru the gears on your hottest/driest (most likely to ping) day no matter what RPM that makes the slots max out at. You want the curve to start (point x) several hundred RPM above your idle speed in drive