I was hunting the web for some dome info and found this website
Dome Calculators
Its got some useful bits for those that want to work out the finer points of their designs ! It does a lot of the math for you without the effort. What is good is the 'Level' field, which gives you the dome radius at whatever interval you request.
It is unit independent, so you can put your calcs in as millimetres, inches, cables or whatever you desire.
Cheers
Peter