So here's my current setup:

F: 18x8.5 +45 tires: 225/45
R: 18x9 +50 tires: 255/35

Not happy with the small rear lip and would like to go bigger (my track days are over, so she's only coming out to car meets from now on)
Im not sure how many inches my lip is or where to measure from... I was planning on putting the rear wheels in the front and getting a bigger lip for the back wheels. I don't mind changing tires if necessary. The car is not a daily, on KWv2 coils, so I don't mind running negative camber.
1/4s are cut (bumper tabs not relocated, but willing to) and fenders are stock.

Can anyone tell me what size lip I can buy (2", 2.5". 3"?) and what size tires I'd have to run to get a good fit with little to no rubbing (


Not sure if this helps, but this is the rear wheel (tires are now 255):

 

Well you are going to be the best judge of this ultimately, since you get to stare at your fitment every night before you go to bed, but based on my experience with your level of fender modification, you can fit 1/2" more lip in the back on the same size tire. Obviously the current 255 will look slightly more stretched if you want to call it that. This would be taking your current 9" 50+ and effectively making it a 9.5" +38 which you can get many examples of in the rpf1 fitment thread as its a popular aggressive option. The other option to get more stretch on the 255 is to change the offset and width to a 10" +50. This will look the same lip width on the outside as the 9.5 +38 option, but you will have another 1/2" of rim width on the inside, helping to stretch the tire further for ease of fitment/looks.

 

Just by "re-lipping", the following sizes will be possible (offset rounded),

F: 18x8.5 +45
-> 18x9 +39
18x9.5 +32
18x10 +26
etc.

R: 18x9 +50
-> 18x9.5 +44
18x10 +37
18x10.5 +31
18x11 +14
etc.

As for lip measurement, dismount the tire and simply measure the size with a tape measure or ruler, the measurement you want will be the distance between the "bead" (the same point from where you measure total width) and where the inner and outer rim shells meet (where the line of silicone is). Since you didn't provide us with disc type, it's tough to estimate what the lip size is on your wheels, and the best way is to measure them, to be sure. Once you have that info, you can reference the possible sizes listed above.

18x9.5 +32/10.5 +31 would probably suit the "flush" look quite well.

 

^Based on this I would leave the fronts alone and have the rears redone to either the 9.5 +44 if you don't want to have to flare the hell out of your fender and 10 +37 if you are willing to do that. Any more you will have to run very stretched tires and an insane amount of camber.

 

This is what i did when looking for new lips for my VS-XX.

Just find your wheel size and offset here, and then just move it diagonal from your original offset/lip size. eg. Down and to the right if you are increasing lip to the next size .

http://workwheelsusa.com/media/wheel...-XX%20Spec.pdf

It's basically half inch increments (12mm-13mm) as you move from one lip to another, but it will also add that to the width

No need for fancy calculations and or having to measure, you'll know exactly what lip to order (assuming you have the correct width and offset of your wheel).

Your fronts have a 50mm lip (aka 2inches) but I can't seem to find a 18x9 +50...

 

I would like to add to this for the sake of clarification,

Adding one inch of "lip" will lower your offset by 13mm (12.7 but rounding up for the sake of simplicity). Likewise, adding one inch to the inner barrel will raise the offset by 13mm. By adding one inch - I mean replacing the part with a one inch wider part.

Likewise, reducing the width of the inner will lower the offset, reducing the width of the outer will lower offset.

When referencing the Work sizing chart, knowing the disk type is crucial, and many sizes have been discontinued over the years.

The best way is to measure the wheel components (just a measuring tape will suffice, Work never used 1/4" increments, so just round to the nearest half inch). Just like how you would measure the width of the wheel from "bead" to "bead", but measure from the "split" (where you will see a strip of silicone) to the "bead".