Hi there

On voice, I was always told that your signal coming back should be no stronger than the satellite beacon.  Not always easy to gauge, but a good starting point.    If you follow the first rule then radio power output will be dependent on the antenna used to transmit.  Obviously,  if you are using a pair of crossed dipoles then you will need more power than if you are using say a large  X-pol yagi. 

I guess this doesn’t quite translate across to data modes,  but If the satellite hears you and responds then that is at least enough power, so try reducing it until it stops working for you.

Cheers

Wil. G6WQN