Explain a bit better how you are running your GT800 - it seems like you are describing that you are running it in 2 channel (stereo) mode but only using one channel's output to the Cab IE it's a mono 120W 16 Ohm cab .... is that correct? If so, then the maximum output it will provide @16 Ohm per channel is 120 W - so basically you could turn the channel level you use to feed the cab up to 3/4 full open and turn down your Axe output rather than running it full open.
Your one point of playing volume adjustment would then be the Axe Output level knob and not the Matrix level knob(s). Running the Axe-FX output full open is putting out a very hot signal I think!
The reference Matrix led guide charts can be found on their website
here
I've set things up on my Axe/Matrix combination as below:
I have a GT1000FX and my cabs are rated 200W @ 8 Ohms. I run the amp in 2 channel mode with a channel going to each cab.
I set up things so my loudest boosted preset is hitting around +4 on the VU meters in the Utility menu and not clipping the output led on the Axe. (0dB in the VU utility menu is only a marker to help visualise things when volume leveling your presets - it's not actually anywhere near 0dB at the outputs - so having peaks of +4 to +6 in them is still safe)
I call 3 o'clock position on my Axe Output level knob that feeds the Matrix as the 'absolute limit' I'd ever dare use.
With the Axe output set to 3 o'clock (my theoretical absolute) I then turn on the Matrix (it's a SS amp so no need to have your cab connected at this point unless you want to get deaf) with it's levels down and gradually bring them up until the leds do the right thing while playing as per the
Matrix website charts for my cabs ratings.
For me that means I can run the red leds safely to where they are practically solid on with just the hint of pulsing and still be well within the ratings of my cabs - this equates to somewhere between 2 and 3 o'clock positions on the Matrix levels knobs - I reckon I'm putting out about 80W @ 8 Ohms per channel at those extreme Axe output level settings.
I then turn down the Axe Output level and plug the cabs into the Matrix - then I bring up the Axe output until it's loud enough .... and believe me that's well before I get anywhere near the 3 o'clock 'limit' position.