The trick is you do not want to bottom out your scope, what you want for good repeatability is 1/3 to 2/3 of the min and max, example if you have a scope with 60 min of elevation you want to be zeroed at 20 min and then if you need they extra 20min you will be at 40 min with 20 min left at both ends, if you keep it in that sweet spot it should always hold and return to zero, if that is not the case then get a 10 or 20 or even a 30 min base to find the sweet spot, I say this due to the fact that most scopes except for high end ones do not like to be bottomed out, they tend not to return true to what ever you set your zero at,

Tanks Dean