Sorry if this has been asked before but i'm going to replace the covers on my mtc mamba 3-12X44 with butler creeks. The supplied ones are good but because they're not sprung open they keep flapping back. I think i've worked out i need a size 21 for the objective but i'm a bit stuck with the eye one. Has anyone fitted one and what size do i need? The focus ring is throwing a spanner in the works with measuring

ATB
Matt