PER-LOD-WBA-LTS P 1/4. Especially P 4/4.
You should have one of these onboard to calcultate it "in the middle of nowhere", i did it during line training and do it from time to time.
There was a power outage at the airport we were operating out of a couple weeks ago, had to verify the manual loadsheet that they provided. So we did it on our own too.
First the MACZFW is calcultated by a computer or manually using the above method, then the MACTO is calculated using the fuel onboard. Some FMS will give it to you once you enter the MACZFW and FOB (MACTO), some won't so you either have to do it yourself manually, either enter it using the computerized loadsheet.