So...am I correct with the -12mm offset equals a 4.5" BS on a 9" wheel? Because that is what I ordered
I think 4.5 BS/ -12 offset will likely work out well for you. BS/offset changed sometime around 03-05, so is different than the earlier trucks. I have a '00 Excursion, so similar year & platform to yours & initially tried 4.75 BS with 17 x 9 (different diameter to you, but same rim width, so same principles apply), and with a Toyo MT 17x13.5x37 I rubbed on the inside against the leaf springs. With a 12.5" width tire instead of the 13.5" tire, I might have cleared on the inside with no rubbing. But 9" rim width, 4.75 BS with 13.5" width tire had more rubbing than I was comfortable with & I didn't want to modify my end stops (e.g. by welding a washer or nut on them) to limit side-to-side steering movement to prevent rubbing. So I went to a 17x9 4.5BS / -12 offset rim & feel that is the perfect balance. I still have very minor rubbing at full steering lock occasionally with new 13.5" width tires, but it's very livable, and would likely not rub at all with 12.5" width tires. As far as the outside, my tires stick out a hair beyond the fenders, outer edge maybe even or an inch or two outside, but it looks & works fine, it doesn't look ridiculous, like e.g. if half the tire was inside the fender & half the tire was outside. With 12.5" width tires, i.e. 1/2" further away from spring on the inside & 1/2" less sticking out, it would be even better, if one's tire of choice is 12.5" width. Since 325/60/20 works out to 20x12.8x35.4, I think you'll be fine & made the right choice.
P.S. As you might have figured out by now, one might originally suspect that 9 rim width / 2 = 4.5" BS would mean 0 offset, but that doesn't take the finite thickness (1" ?) of the rim hub center into account, and since offset is measured from the inside hub surface that contacts the hub, the actual offset is half an inch offset different from simply dividing the rim width in half.