You’ve bought your rifle and scope so now you need to mount it. You want the scope as close to the barrel as possible but what height rings do you need? Calculating this is easy, ridiculously easy and can be done with no more than the loose change in your pocket. How? Pay attention!
Set your rifle solidly upright. Ideally, clamp it into one of those cleaning stands or a vice. Make sure the rifle is horizontal. Take your scope and position it on top of the action with the ocular lens (eyepiece) the correct distance from the back of the stock to give you the eye relief you need. Average will be 280mm. You cannot sit it directly on top of the action yet because the objective lens hits the barrel. You need spacers under the tube to lift it clear of the barrel. Anything will do but ideal is the fore mentioned loose change. Coins are convenient and come in different thicknesses. Put a stack in front of the adjusting turret and a stack behind. Adjust the number of coins until the objective lens is clear of the barrel, measure the height of the coin stack and you have the height you need for your mount.
A couple of points to note. Some actions are a different height front and rear so you need to use different stack heights to keep the scope parallel to the bolt or bore line. If your rifle already has a Picatinny rail fitted this will already be correct and you just need to sit the stack on top of that.
Scope mounts in Europe have a stated height for each component, known as the ‘BH’. If measuring from the top of a Picatinny rail you only need the height from the top of the rail to the underside of the scope tube to give you the ring height. If measuring from the top of the action you may need to add mount component parts together. For example, if you measure from the top of the action a height of 14mm and plan to mount a Picatinny rail you need to subtract the rail thickness to give you the ring height you need. A typical rail will be 6mm so your ring height will need to be over 8mm otherwise the objective lens will touch the barrel. If you use lens covers you need to add a bit otherwise you will not be able to get them on.