250 = 2 x 5 x 5 x 5.
90 = 2 x 3 x 3 x 5
So the 90:1 worm has the first two factors of 250: 2 and 5. You need two more 5s or 25. Those two fives can be in a 25 or 50 division plate. Or you can use some other scheme to get them.
Most sets of plates tend to have numbers that introduce additional prime numbers to the mix so you see things like multiples of prime numbers like 7 or 13 (3x7 = 21, 2x13 = 26, etc.) So even if you have a set of plates, you probably do not have a 25 or 50 division ring. There are several ways to make a 25 or 50 division plate if you don't have one.
You need to understand that if you use a hole ring on a plate on a indexer or RT, any errors in that ring of holes will be REDUCED by the worm factor when you use it. So if your hole circle has errors of 1 degree, a big error, then the divisions made with it will have errors of no more than 1/90 degree or less than 1 minute.
You can lay it out by hand and just drill as accurately as you can. With care you can have errors of less than 15 minutes (1/4 degree) in that plate which, when you use it, will give you an accuracy less than 8 seconds. That is probably more accurate than your worm.
Another method of making a plate with hole circles would be to make a scratch plate as accurately as you can (above) and use that plate to make a second plate which will be 90 times more accurate. If you are going to use the plate on the indexer or RT, there is little or no reason to carry this process any further.
So, the answer is YES, you can make a 250 division part, but it will take some extra effort.
This will work for ANY number of divisions and for any worm ratio. You can, with a little work, have hole plates with any number of holes that are just as accurate as your worm gear is. The magic factor is having a good, high quality worm gear set.