When I set up the 727 for a flight, I add 20kgs/40lbs per passenger as luggage.
Then. After adding enough fuel for the flight, I check to see how much extra cargo I can carry without exceeding Maximum Take Off Weight or Max Landing Weight.
I divide the total by the total ULD's the plane has and add that amount in the box in ACE.

If I'm flying one of the Freighters. I use the same principal (divide the total by the total ULD's the plane has and add that amount in the box in ACE) for MTOW and MLW.

You might find the info you seek over at the Boeing web site (more specifically here: http://www.boeing.com/boeing/commercial/airports/plan_manuals.page and select the 727 from the list). There's a lot of the info in the PDF's, that you can download, that are very technical, but there is some helpful info within them nonetheless. ;)