Using a tape measure and pictures of other peoples setups.
Offsets in the +3Xmm's had too much poke for the tyre size I wanted.
Also in Australia you can increase your track width only by 50mm (therefore 25mm per side), so increasing the tyre size 20mm in width (285's) gave 10mm per side and the offset of +45 (original +55) giving another 10mm per side, therefore legally the max would be an offset of another 5mm with my current tyre size.
The max offset I could therefore go was +40mm anyway.
Hi Darby
Track is measured from the inside of one wheel to the outside of the opposite wheel, so effectively the centre of one tyre to the other.
the width of the tyres don't affect track, so in Australia you are legal to an offset 30+ but poke off course is a different matter.
Your Grenadier looks fantastic, I have a set of Black Rhino Yellowstone wheels 17" 38+ at home, just waiting for the grenadier to arrive to put them on.