I've shortened a few magazine followers in my time, and must advise you to remember that you must take off a bit more than what is needed to squeeze the next round in.
There must also be enough room for the bottom of the slide to press the top round down into the "before popping up to get stripped by the slide" position.
M14s and M1As with USGI magazines, for example, are strictly 20-round propositions, not 20+1. A fully loaded mag won't fit under the closed bolt.
Technically, you should also have a bit more wiggle room so if the full mag is pressed upward inside the mag well, there will not be enough drag from the top round to make the slide short-cycle. So, measure room for the last round, room for the closed slide/bolt to depress the round in the mag, a little more room for where the floorplate tops out in the mag well, plus about 1/16-inch to be safe.
Ram-Line's super high-caps got that way by using a flat ribbon spring attached to the top of the mag next to the feed lips. Loading the mag unrolled the coiled ribbon like a party whistle unrolls its paper tube (whaddaya call those things?). That design eliminated the need to include room for the compressed spring under the follower.