Electronic – Creepage distance over pcb

Here is a diagram that should help: -

Picture taken from this document.

Please also note that creepage distances might have to be bigger dependent on your pollution category and your material group. See this document for further info.

There is also this calculator - I have tried it and for 250V AC, the protection distances seem to be right i.e. basic creepage is 2.5mm with 2.5mm for supplementary making a 5mm total for reinforced. Ditto the clearance figures - they add up to 4.0mm for reinforced: -

Pollution degree is an empirical characteristic and not a real physical quantity, so no, there will be no strict analytical formula. The next best thing is to take a table with creepage distance values for different pollution degrees and approximate these values with some kind of function (usually a polynom).