I have 42mm scopes and use the low. That's what Euro-Optics recommended.
What kind of scope and action length of rifle? Does not sound even remotely correct.
308 Classic short action w/ Leica ER 3.5-14x42 scope
300 Classic long action w/ Leica ER 2.5-10x42 scope
375 Classic long action w/ Leica ER 2.5-10x42 scope