Probability is calculated on the ovum rather than the individual; meaning that an individual may potentially be a surrogate mother of a different species, the compatibility is calculated against the ovum to host and the ovum to breeder combined.
The maximum compatible probability is 100%; there are currently no boosters. The order in which probability is calculated and how is as follows, and stacks with each layer: (i.e. calculation for each of the numbered items below (except #4) if the values were 50%, 90%, 150% respectively; the total compatible probability of conception would be the base of 100% multiplied by 50% * 90% * 150% = 67.5% total probability)