I was told that there is a certain number of %MF that if worn while doing Hell Meph runs will guarantee godly drops, I was told that it is an exact number, and that it only works with meph, is this true? and what is that exact number?
As the above posters have mentioned, there is no "magic number" for MF. What you heard was false.
Some MFers are ... shall we say 'superstitious' in these regards. There are those who subscribe to the myth that exactly 250% MF (no more, no less) will guarantee the best drop results. Humorous, but untrue.