I'd stream at 1.2Mb with 1.7Mb upload available. But the true limit will be found by you experimentally. Vary your settings up and down a bit (buffer too) and see if the OBS logs indicate any problems sending the stream, and if your game performs worse (pings getting worse, lag spikes, etc). If there's voice comms in the game, see if you get lagged out when a lot of people are talking, etc.