What I have is a Leupold vari x 2 3x9. Looking to put on either a 257 Roberts or 35 Whelan. It has different yardages on the outside of the objective lense. If I set at 100 is that ok?
Setting it at 100 is not a bad sort of an idea, but the way to do that has nothing to do with the markings on the scope. Start by setting the
eyepiece focus first. You do that my aiming toward the sky, ideally in low light (where your pupil is more dilated and hence better able to pick when the reticle's focused). Adjust the
eyepiece, looking away frequently, until the reticle is as dark and sharply focussed as you can get it.
Then, using your maximum magnification, aim at a target 100 yards away, and adjust the
objective focus, again looking away frequently, until the target image is sharp. You can confirm that you have "dialled out" parallax at this distance by moving your head around behind the scope without moving the rifle (you need a steady rest for this) - parallax will show up as movement of the reticle with respect to the target. You need to adjust the objective until there's none of this movement. Once you have the objective adjusted such that there's no parallax the scope is parallax-free at 100, regardless of what the graduations might say on the AO.
For most of your hunting, especially at magnifications at the lower end of the scale, you can forget about it after that. You'd only really notice the difference in short-range precision match shooting, or perhaps at really long range, and even then it won't amount to much if you have a consistent eye position behind the scope.