Yes, those simple formulas are only good for very approximative results. Stay away from them. You can have the boat fully measured and weighted and get the results to someone that has a very expensive program (Naval cabinet) that makes the calculations for you (you have to ask what they need) or you make the easiest way that is through an inclining test following ORC instructions. Contact ORC for instructions.
About the differences you can get on stability curves using different programs you may want to have a look here:
Misleading stability - Yachting and Boating World Forums