Do you have the rifle and the scope in hand? The top of a tikka is flat. The talley website gives you the thickness measurement from the top of the receiver to the bottom of the scope. Stack up some pennies/dimes/nickels (2 stacks) to the height you want. Tape both stacks over the receiver where the rings would go. Shoulder the gun and have someone slide the scope to see where you would want the eye relief to be. Tape the scope over the coins about where you wanted the eye relief, you have now mocked up your mounts.
extra lows .300" under the scope to receiver
lows .400"
medium .500
These are for 1" rings. I think the same applies for 30mm
Since the Tikka is flat, it is very easy to machine a little off the bottom of the rings if you want your scope to sit lower.
If you have a scope mounted with lows, and more than a .100" gap to the barrel, switching to the next lowest will drop you exactly .100"