the ruler can be any length. the angle is determined not by the length but by the degree between the two lines where they meet at the center of the circle. let's see: if you have a plate and you divide it in quarters, then where the lines meet in the center would create 4 - 90* angles. if you divide those lines in half again, you get 45* angles, etc.
when added up they will total 360*. when divided they become 4 - 90* (or 8 - 45*'s or so on) degree angles. with me so far?
now, if you do it with a little saucer, it will work. if you do it with a huge round platter, it still works. that's because the length of the angle (ruler) doesn't matter.
i have a long ruler to accommodate any length. i simply tape-mark a shorter distance on the ruler if i need to.
edit: for testing purposes, remember that there are 360* in a complete circle. how ever many wedges you want to use MUST add up to 360*. that should help you determine how many degrees you want your wedge ruler to be. decide how many pieces you want to cut and sew.
edit edit: i see what you mean. the measurement at the tip of the wedge is zero. the measure at the wide end is determined by the length, which is always different, depending on how far out you intend on using the ruler. how far out ill you be using it?