Hey guys I'm new here.

I've got a Mac Pro with 2* X5690 3.46GHz 6Core 12Threads processors and a AMD RX 580 8GB.

I'd love to donate it's idle time to SETI@Home but would love to do so in a efficient way:

Run during my cheap electricity hours.
Not run 100% of GPU and CPU because noise/heat.

How could I set SETI to only use 33% of my GPU?
How could I optimise CPU usage: what is better? 33% of the cores at 100% CPU Time or 100% of the cores at 33% CPU Time. Etc.

Thanks in advance guys.

You probably know already that you can set the time(s) of day to use your computer under the Advanced View / Options / Computing Preferences / Daily Schedules.
And the processors to use in the same window, but the Computing Tab.

If your end goal is to use 33% of the total computer for the day. It would likely be something like 50% of the day (for power use) and around 66% of the CPU during that time.

Mike is the master of AMD command lines (tuning for the cards) he likely has something up his sleeve to be an efficient performer with ~70% use to give you. Hopefully he can give a pointer there.

EDIT: Modern GPUs by nature are more efficient than CPUs for the amount of work done per watt used. So favoring GPU use over CPU is more efficient - up to a point where it may become too loud for your liking.

It's just finding that right balance that you feel it right for heat and/or noise. But it's winter time, so free heating :D