Pythagoras, that famous Greek groundschool instructor said:-
- Take TAS and square it
- Subtract the square of the climb rate (in the same units).
- Take the square root of the answer, and that's your equivalent TAS in level flight.
- Then factor as usual to give groundspeed.
(Incidentally, and this has nothing to do with pythag, if you are trying to be really precise, convert from IAS to CAS and work out TAS from there - there should be a graph or table for this in the POH somewhere. The difference can be as much as the greater of 5% or 5 knots in a certified aeroplane).
G