Calculating the magnetic flux density in cylandrical coordinates near a neodymium disc magnet
The use of finite element analysis is the easiest approach.  If you are a math guru, here is a suggestion for a non-finite-element approach which would seem reasonable for a thin disc magnet: assume that the magnet is actually a group of magnetic dipoles, the effects of which are additive.  You could then calculate the fields from the dipoles along on a radius, then rotate this result.