Likely hood of getting pregnant from precum if i was ovulating and took the plan b pill less than 24 hours later?

Not likely: Plan b is very effective if taken in first 24 hours. Pre-ejaculatory fluid can cause pregnancy but not with a high percent. Please make sure you use better contraception. Plan b (is literally plan b). Make sure you have plan a: e.g. Birth control pills, condoms, spermicidal jelly etc.

Very low (not zero): While the chances of getting pregnant in that setting is low, it is not zero. Neither the "precum" or the Plan B is 100% effective, as there can still be sperm and the Plan B doesn't always work. Other forms of birth control will be safer if you are having sex frequently. Of course, Plan B won't protect against disease, so using condoms even if you are on something else makes sense (to me).