Base ISO isn't a matter of "Calculations" its the performance of the sensor. The Native ISO is the ISO that has the most dynamic range, it's that simple.This could be difficult to find out since manufactures really do not like divulging the base iso of their sensors\cameras.
If I get a chance this weekend I will try and do some calculations based on some covered lens shots, compression, file size and a bit of maths...... I will let people know my findings but the results will be a best approximation.
if people really want to know it its easy to test. it doesn't require calculations and really if you are using the cameras professionally you should have tested for it anyway and if you dont know how to test for or dont care you're not a professional and shouldn't be charging for imagesWhich can be worked out (since the manufacturer doesn't quote it)
Your ISO is important and will be the "best" at different levels in different lighting. Your ISO could be the best at 200 during the day but need a higher ISO at night or on cloudy days.I don't really know any of the technicalities behind this but I read somewhere ISO 200 is the 'best' for the X3.