It's pretty easy to do but it can be a bit tricky when you have fuel loads that change the CofG as the are used. A lot of the programmes that I have seen have static CofG positions that are picked off of a graph whereas mine are completey dynamic ie: it doesn't matter what your fuel state is - you get the correct arm/moment. This basically replicates the POL/REC function as found on some calculators - I can give anybody who wants it the basic module.