I have a Beetle based classic trial car and replaced one of the inner track rod ends (steering box) I was careful to measure things up before I removed the old one and fitted the new one. However (predictably I suppose) it is slightly out, scrubbing the inner edges of both front tyres. Measuring the track front and back of the wheels at hub height on the wheel rim shows the measurements to be virtually identical. Without resorting to a garage to get it tracked can anyone hazard a guess at the difference I need between front and back of the wheels. Am I right in saying I should have some toe in? Always a mystery to me as are electrics as well!!!!

Try 3mm toe-in for starters. That should be roughly 0.5degree. All depends on the wheel size but roughly 1mm = 10', 6mm = 1deg

It sounds like it has toe-out the minimum setting for wear and rolling resistance is parallel in the normally loaded condition but toe-in is usually specified to allow for lost motion and to provided less "nervous" steering response around the straight ahead position and under braking.
Vw type 1 factory toe in setting is 0.5 toe-in.

The best quick way for DIY itracking across the axle is a Gunson Trakrite G4003 you just need to drive over it carefully and take a reading.

String and box alignment method probably just as accurate as any - if you can be a*sed and have flat floor.
Actually pretty simple once you've got your head around it and costs next to nothing (true locost).
Use braided, flourescent fishing line from local tackle shop.
Mine knew exactly what I was after saying "there's a well known car maker in Woking buys it from me"!
HTH.
Cheers, Pewe10