Hi, Thorns.

I think there are two ways of looking at it.

1) PRICE - If a horse pays $1.50 on Unitab then a 20% increase on its price comes to $1.80 ($1.5 x 1.20).

2) LIABILITY - In the above example the TAB liability is $0.50 whilst a 20% increase comes to $0.60 which translates into a price of $1.60.

I don't know which method is correct. I suppose it boils down to Betfair's price being closer to $1.60 or $1.80, or even maybe the average of them being $1.70?

However, I would assume each $1.50 Unitab horse can easily differ on Betfair depending on circumstances, so there may not be a conclusive answer?

Its quite possible the results of my testing may not be as good as hoped.

After saying all that, using your method would make me feel more confident, but we'll never really know what prices we would have obtained when testing a method.