if you think you'll be using it over and over, then get the ruler. but remember the ruler comes in all *'s. so, if you want one that makes a 12-part plate, that's one ruler. if you want to change to a 10-part plate, that's a different one. and so on. it can get pricey.
so, here's what you can do for free. draw an oversized circle.
find the exact center. it has to be precise. the best thing to do is to draw the circle with a compass. that gives you a perfect circle and the exact center (the hole). draw a line through the circle passing right through the hole, using a fine-line pen or pencil. measuring with a 90* square, mark another line through the circle, right through the center. you should have 2 lines forming a cross, with the ends going straight out the circle.
now, carefully, measure off 2 dots on the circumference of the circle between the cross lines. draw lines from the center 'hole' to the outside 'dots'. that will give you 3 segments that will each be the same exact size. if you do that in each quarter of the circle, you will end up with 12 segments, all equal in size.
using the same method, you can divide the circle into any number of sections. remember to add seam allowances. trace and recut in mylar.
to my way of thinking, it allows changes in the number of petals you might want to use in other projects, depending on how many fabrics you want to showcase.