First, you need to know that it's not likely to help you if you take it less than five days after an imitrex injection. This is common belief here -- maybe you will turn out to have a different experience. Also, taking a "very small dosage" once probably won't help. Read the numbered files in the ClusterBuster Files section of this board to understand the standard protocol, but basically it's (1) 5 days off imitrex; (2) a recommended dose of roughly 1.5g of dried psilocybin mushrooms, taken every five days.
If you're going to do the D3 regimen, you should do the whole thing. Read about it in the ClusterBuster Files section. Take the calcium about 8 hours away from the verapamil. Also, if you are going to continue with imitrex, you should read the file, "Extending Imitrex," which is also in the ClusterBuster Files section. You can use half to a third as much imitrex to stop each attack. (Many people are convinced that imitrex/sumatriptans extend CH cycles and create substantial rebound attacks.)