I had exactly the same issue with my Harrison (determining back space). I would ask people what back space they had, then look at their car and try to compare that to mine along with the tyre size I wanted.
Knowing I had one chance to get it right, as I was getting the wheels and tyres from the US caused me a few sleepless nights.
In the end I borrowed some wheels and had the whole thing wheel aligned by a reputable suspension place. Good thing I did as it altered the whole stance of the car. They had to add a number of shims to the LH front and the rear needed a fair amount of work also. This actually gave me more problems as I found the body was off, were as before I was trying to kid myself everything would be OK.
I think the money I spent on wheel aligning was one of the best value things I did. The guy actually let me leave it on the hoist and i fixed a few other things at the same time.
Then I went home and using a plumb bob hanging right through centre of axle I measured from the lip of the guard to the wheel hub and then spent a bloody long time ensuring the wheel manufacturer was absolutley clear with my wheel backspace. He supplied the drawing in fact.
Result bolted up the wheels and spot on.
Could extend the guards like Venom.
Bruce