Another way to work the difference is to convert it to fractional odds first ,then reduce the Betfair odds by 5% by multiplying it by .95

This adjusts the true comparison figure by the 5% BF com.
e.g.
BF (2.24) 1.24/1 x .95 = 1.17/1 (2.17)
TAB (2.10) --------------1.1/1 (2.10)
Diff .07 in fractional odds.
6.36% Diff

BF-----(3.70) 2.7/1 x .95 = 2.56/1 (3.56)
TAB---- (3.40) ---------------2.4/1 (3.40)
Diff .16 in fractional odds.
6.66% Diff

Now you have a price comparison to look at when placed next to the TAB prices.
Remember that one has to convert to fractional odds first to make it work.

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Cheers.