After bleeding, if necessary, you should be aiming for about a 20 degrees F (10 degrees C) drop in temperature from the input pipe to the output pipe. Open both the input and output valve (thermostatic or otherwise) fully and with the pump running measure the temperatures on the pipes (you can get clip on thermometers) then close down the output pipe valve a little bit, leave for a while, say 15 minutes and see what the temp difference is and gradually keep closing down the output valve till you get near the difference stated earlier. Hope this helps, Cheers Andy