I generally recommend setting your bitrate at around 80% of your upstream, (up to the limits that Twitch/Beam/Youtube ingest servers will accept) for better stability of your stream and system access. (i.e. to still be able to use the internet for other stuff if needed while you stream).
With a 1.25 upstream (And you may want to test this a few times, especially at different times of the day/week to get a good average), you might try 1000, even though that is pretty low. You could try 1100-1200, but you may run into issues and frame drops.
With low bitrates, it's important to to reduce the image size and framerate of your stream, to get a decent quality. (1080p60 has a LOT more pixels that need to get crammed into the same stream as 720p30, so more compression). If you haven't already tried, you might try setting a smaller output resolution in the settings.