This pic should help to explain. Basically, how much of the bottom magnet parts chain sticks out over the angled kicktail comes down to:
1. How long the kicktail was on that particular board
2. How far forward/back the bottom magnet parts chain is placed on the board.
Ideally, you want the kicktail to be whatever length, and the parts chain to be shifted as far forward, as will allow only the flat portion of the rear-most bracket (which begins immediately behind the rear edge of the rear circular magnet base) to be angled upwards.
If your kicktail is longer than average, you must compensate for this by shifting your entire magnet assembly parts chain forward a bit. If your parts are positioned a bit too far back, you had better not have a long kicktail or you'll have part of the magnet base itself projecting out over the kicktail.
Notice how on Bob Gale's Almanac-burn-scene HB the kicktail start point is way too far forward in relation to the magnet parts. To prevent a gap between the magnet base and the board, the vaccu-formed magnet base is deformed downwards at the rear to follow the kicktail—not ideal, and probably not possible with resin parts unless they were heated. Even then, it just looks wonky.
Now look at this Ketzer board. See how the kicktail is shorter, putting the kicktail start point JUST BEHIND the base of the magnet as it should be, and as indicated by the yellow dashed line in the pic above of Bob Gale's board? That's what you want.