If you're not selling based on the ink and paper (which I agree with), then it is a little silly to make your prices based on "markups" at all.
If you're conceptualizing it as "Paper + art" then you should just add a flat fee for the creative art cost to whatever the paper is, not try to confusingly figure it out as a scaling percentage.
So for example, the artistic and equipment effort that goes into one pose might be $200 for you, or whatever.
if it's a 8x10, then the price is $200 + the paper and ink for an 8x10 + fixed markup on paper only (for shipping etc.)
if it's a 24x36, then the price is $200 + the paper and ink for a 24x36 + fixed markup on paper only.
if they want to buy two of the same pose, one in 8x10 and one in 24x36, then the price is $200 + the paper and ink for a 8x10 + a 24x36 + fixed markups for paper only. Or $200 on each print (stingier, and kinda ridiculous, but at least justifiable by some logic)
I'm not saying that's the only way to price it. But if your philosophy and reasoning is that you're charging for "art and paper separately" then flat fee is the logical conclusion, and as a customer, that's the only thing I would expect without it seeming fishy if your explanation was "art and paper separately"
or if the contract is such that it says I am purchasing X many photos with my original shoot fee, then i would find it pretty questionable to charge an artistic fee for any prints at all. I would expect my original fee to have been the art fee already (what else is it for??), and prints should just be price of paper + small markup for handling and such, on the order of 50% ish.
If you're charging for art by the pose in prints or digital copies, then i would expect the shoot itself to have been almost free (a small "deposit" charge makes sense, but should be way lass than regional rates for people who just give you a CD of all your images).
Charging people twice for art is something I routinely advise my friends to not accept when they are shopping for photographers for weddings, etc.