Diopter adjustment lens?

Hi all, I've got a slight problem. My d3100 viewfinder focus dial can adjust to give perfect focus for my vision but needs to be fully rotated anti clockwise to achieve this. My d5100 however at the same extreme is not quite in focus. I know you can get diopter lenses for the viewfinder but from what I describe which one would i need to get that little extra?

It's very frustrating that the 5100 is not sharp when focusing. Pictures are perfect and live view is sharp so it's just viewfinder.
Thanks

I don't know - it does seem puzzling, because the D5100 manual (page 206) says it has slightly more diopter adjustment range than the D3100 manual says (page 176).

Says to test it before purchase, so a local camera shop seems appropriate to do that.

I've read some reports that the 5100 adjustment isn't as quoted on some bodies. It doesn't stop me using the camera but makes the odd occasion where I use manual focus more difficult as I have to use live view

I would get one that is appropriate for your vision rather than one to just get you past what the dial can provide. If your eyes continue to get worse as you age, and they all do, then you have more room to adjust it.

I was googleing but can't get any idea how it looks like> Mayb eI need some too.