I always try to set cantis so the lower part of the arm (mount-to-brakeshoe) is coming close to vertical as the show his the rim. It isn't always possible, but there is room for you to withdraw the shoes, and let the arm swing in farther. This, combined with a proper length yoke will give you maximum leverage. Plus keep the show bolted close will improve rigidity which can be very important in smooth effective braking.

As for centering the brakes. The bosses of the ere when these were made usually had 3 positions do the spring's leg, and you could coarse adjust that way, then fine tune with the shoe position. Or you could work the spring as we often had to to get them dialed in.