This is 1 way I use to calculate the bare minimum height required then I add 5mm to the number to allow a little clearance. If using bases with built-in elevation ill add 15mm but the way I use most is to buy a couple sets of rings then return the ones I didn't use
X = Min base height from rail to bottom of the ring assuming no elevation is included in the mount.
For a 56mm objective on a 30mm tube
X= (56/2)-(30/2)
28-15=13mm