If you look at the contact breaker end, cover off . . .
. . . . When the heel of the opening point goes onto the lower lobe, around 5 o'clock typically, it is firing the REAR pick-up of the mag - g'box side.
The lobe at 10 o'clock fires the front pick-up - engine side. 'Nearest to nearest' as it were.
So, if you do your set-up using the lower lobe, then the cyl that is on the compression stroke (both valves shut) is the one you hook up to the rear pick-up, g'box side.
Some people actually care which HT lead goes to which plug. I try to have the rear pick-up go to the RH cyl, but dunno why really!