Pre-Calculating Large Tables of Values

I’m currently working on a project where we want to know, based on a euclidian distance measure, what is the probability that the value is a match to the another value. i.e. given an actual value, and a theoretical value from calculation, what is the probability that they are the same? This can be calculated using a chi-square distribution with one degree-of-freedom, easily enough by considering how much of the chi-cdf we are taking up.