Lower Wattage Cabinets with PS100

I want to better understand how the output power works with cabinets or speakers that are rated under the 100 watt rating of the PS100. Is the output power relative to the volume position on the PS100?

I chose a PS100 because of the channel and effect loop switching. I also thought the clean extra headroom would be better to preserve the original amp tones. I have a single 12 cabinet with an Eminence Red Fang (30 watts RMS/60 watt peak), and a 2X12 cabinet with a Celestion Vintage 30 (60 watt RMS) plus a Celestion Greenback (25 watt RMS) which I guess is 85 watts combined. Both of my cabinets are under the 100 watt power output of the PS100. (I also have a 15 watt 10" Eminence in a Bogner Duende combo which seems like a bad choice to pair with the PS100).

Since both of my available cabinets are under the 100 watt power output, I want to be careful to not push too much wattage to either respective speaker set. Is half way up 50 watts, etc?

Thanks for any help you can give me in understanding this.