That sounds about right!
1. Test car on RR - note ATW figure;
2. Remove engine, set-up on dyno, note ATF figure;
3. calculate difference.
4. remember, everytime you change the tyres, the oil or just about anything else, you need to repeat 1 and 2.
<RANT>
I reckon this ATF stuff is all a con. The manufacturers tell us that a 172 has 170bhp, but weve no way of testing whether an individual car really does produce this figure. It would be a whole lot better if everyone stuck to ATW figures that we could actually test!
</RANT>