Sounds like you've got some good experience behind you and have found an equation that seems to be working, so that's a real bonus. How you would get an idea on retail, that's not an easy one at all. If you're not familiar with the products at all, researching them will be a little difficult. Just in the making sure your measuring apples with apples type of thing.
Some of those past margins you've been hitting have been quite impressive, but you'll know as well as I do. When it comes to truckloads, there is always some gold and always some garbage. You're always just wishing for more gold than not :)
Surplus dealers are usually somewhere in the region of 30-40% on retail, but that's a really rough guide. Depends on the supplier and on the products to a large extent.
To be honest, I'm not really sure I would trust any retail figure provided to me, way too much temptation to inflate that figure to make the load sound way more inviting. Of course that's just the skeptic in me lol, but that's what would be playing on my mind.
Maybe work off a very conservative figure, something like a retail margin of around 30-40% and see how the numbers stack up. If you can turn a margin on those numbers, then you would have to think that you would be in with a solid chance.