So I have come up with a new formula to figure it out.
The Gap - A - The distance from your rotor or drum to the inner fender lip of the top of the fender
The Width - B - The width of the wheel you want to use
The Back Spacing - C - From the back pad of the wheel to the back edge of the wheel
Track Adders - Drop Spindles = .375" (3/8) - D - Disc Brakes .375" (3/8) - E
EXAMPLE - 17x7" Wheel with a 5.00" Back Spacing. Our 58' Beetle has a 2" fender gap and we plan on installing drop spindles and disc brakes on it. Will our wheels fit? Let's do the math!
Back Spacing 5.00" - C
Wheel Width * 7.50" - B
Fender Gap 2.00" - A
Drop Spindles .375" - D
Disc Brakes .375" - E
Front Offset of the wheel - X
Track Adder's Added up - Y
Modified Fender Gap - Z
Narrowed Beam Needed - Q
Real Time Offset - RTO
Back Spacing - BS
Additional Offset - *
(B - C = X) ((A - (D + E = Y) = Z
X Y Z
(7.5 - 5 = 2.50) (( 2" - (.375 + .375 = .750 or 3/4") = 1.25" Fender Gap))
Now we need 1.25" of narrowed track on each side giving us a total measurement of 2.50" narrowed beam. If we would like it flush to the out side edge or tucked further in we can use a narrower beam to give us the added look and effect.
(Option 1) ((X - Z) = RTO) x 2 = Q
((2.50"-1.25") =1.25") x 2 = 2.50"
Option 2 = X - If we have 1.25" of (MFG) and the rim width is 7" we subtract our MFG from the wheel width to give us our back spacing needed to fit wheel on the car. That would give us 5.750"""" back spacing needed to fit. If this was not confusing yet here is the confusing part. When you see a wheel list as a 17 x 7 wheel it is actually a (* 17x7.5") because wheels are measured from the inside edge of the bead to the outer edge. So any time you try to do the math always add .500 (1/2") to the back spacing as a rule of thumb.
(Option 2) ((B - Z = BS) + * = RTO
(7" - 1.25 = 5.750") + .500" = 6.250 RTO
If this math has confused you I still do not believe I actually wrote it out. This is why a majority of clients ask me for this info because it is this difficult. But after a while it is common knowledge. I hope this helps.