Sounds like you're learning offset and backspacing. Got it right except for one minor detail.
Offset is measured at the same place the wheel's width is measured - the inside if the bead. That is what the tire mounts against. A 7" wheel is 7 inches from inside of bead to inside of bead. It's a tire to wheel dimension. So a 0 offset (stock) 7" wheel has 3.5" tire either side of the wheel mounting surface at the bead. The WMS is the flat part of the wheels center section that mounts to the hub of the vehicle. Usually at the matching flat surface of the brake rotor. You got all this stuff right.
Backspacing is a wheel to vehicle dimension and has nothing to do with the tire. It is measured from the WMS to the overall inside of the wheel. What's different is that it's the outside of the bead, not the inside of the bead. Nearly all modern wheels are JJ wheels. Which means the bead is 1/2" thick from the bead where the tire mounts to the outer edge of the wheel. A 7" wheel, since it has an outer and inner bead, measure overall 8" wide. That makes the backspacing 4" on a stock 0 offset 15x7 wheel. In other words you can figure out the backspacing by figuring out the offset + 1/2 inch.
In the case of your 0 offset 15x7 3.5 + .5 = 4".
A 4wd and a lot of FWD use somewhere around 50mm positive offset. Since 25.4mm is 1 inch. You are essentially talking 2". A 4wd stock 15x7 has 6"of back spacing because the WMS is moved outward 2".
The opposite direction from where you want to go. They do make wheels with negative offset. The WMS is moved toward the frame of the vehicle. Which would help you get to your desired poke.
Bear in mind, tho, the farther out the wheel is the more of the width of the tire has to clear the outer fender. That's what causes poked wheels to hit the fender when turning. About the only way to decrease any contact is to jack the truck up.
That only works because most fenders are open more closer to the bottom of the opening. Unfortunately there is no cut and dried way to figure out how much is needed. 2 identical trucks will be close, but not the same do to age, wear, abuse, accidents, etc. On my ZR2 my drivers side just rubs a bit at the front and the passengers side clears with 1/2" to spare. But the truck has 201K. Somewhere in a past life the body got moved back a hair on the frame. Could've been in a fender bender at one time.
When you're done it'll likely have a harsh ride, handling will suck because the center of gravity has been raised, and will sling mud down both sides everytime you go through a puddle. Sure that's what you want?
As far as looking cool goes, that look is only cool in some circles and is starting to get old. I remember when bubble fender skirts and lake pipes were cool.
