I think that you are looking at this the wrong way
Your question is how much on top do I need.
You should be asking is how low do I need to go on the bottom end. Can I get buy with 6.5x on the bottom or do I need less.....5.5...4.5....4....3.5....ect.
IMO, you can never have to much on top, and can always dial down.
But on the bottom, if you have 6 x on the low side and need less, you're screwed!!
I decided that I should never need to go lower than 5.5 x on my LR rig.
POOF...enter the 5.5 - 22 x56 NXS!! I couldn't afford the March 5-50x or I would have one of them.