On the contrary, Ron. Offset is extremely useful on its own. This is cecause it sets the centre point of the tyre - which is what you are really interested in. The ideal offset for the P6 rear mudguard area is aready established at 44mm +/- 2mm. This derives from the Denovo tyres which were fitted as an option in the mid 70's. They were the widest tyres ever fitted to a P6 at 205mm at the standard diameter, and even then, occasionally needed the D post fettling to get them not to foul. knowing this you can confidently assess the likely difficulties with various wheel types. From this point on the rim width becomes irrelevant to the fouling issue, albeit must still be selected to suit the chosen tyre size. ie a 7" wide rim wearing 205 tyres will leave the tyre sidewall in exactly the same place relative to the foulling points as a 5.5" rim wearing the same tyre, provided they both have the same offset. The difference is then down solely to the sidewall shpe on the two wheels, on the 5.5" rim a 205 tyre will be extremely bulbous and prone to roll off the rim under cornering, on the 7" rim the beads will be rather stretched and you risk deflation and a very hard ride.
Chris