Robert
I've been analysing the local figures for a couple of years now, and it seems to give a reasonable estimate. You're right about the overcounting due to girls using multiple phones - that's where the 0.7 multiplier comes from. You could maybe argue that 0.6 would be safer. Either way its simply a very rough approximation.
As to your theory that most indies use AW, then you're way off track. First problem is define "indy", in reality few foreign girls in this country are truly independent, most are controlled or "assisted" to get here to a lesser or greater extent. When you start brushing the service its amazing what underlayers of crime start appearing.
However the bigger issue is that most Eastern European girls now DON'T use AW. Most of them travel on ID cards, not passports and AW stopped accepting ID cards as valid ID around 16 months ago. That's resulted in a large drop in EE girls on AW with a corresponding increase in Vivastreet profiles. At present for Lancashire the ratio is around 4 or 5 to 1. In fact for most of January the EE numbers on AW for Lancs were actually lower than the number of Thai profiles listed.
You tend to get tiers in how the girls advertise.
Those on AW are more likely to be genuinely independent and in charge of their own destiny - thats why they have passports.
Those on Vivastreet are more likely to be controlled to a greater or lesser extent, some voluntarily, some not. There is a degree of crossover between the two, but not as much as you'd expect. Pimps don't like paying to list the phone numbers on AW...
Those on Backpage / Friday Ad and similar are most likely to be here illegally and forced. Again there is a small degree of crossover, but much less than you'd expect.