@catdaddyI have no experience with low sugar marmalade, I am afraid. I have used commercial (Pomona) pectin once for persimmon jam, but didn't care for it. I used only a small amount and it made the jam artificially firm for my taste. I prefer to make my own pectin from green apples a la Christine Ferber.
Google also tells me that ponderosas are a cross between lemons and citrons. Very cool. I am pretty sure they'd make a spectacular marmalade on their own! I can't wait to hear what the finished product tastes like.