Press ALT to open the menu.
Select Aircraft, then Fuel and Payload.

What you will see is the airplanes fuel and payload (of course).

This is my 757-200. Depending on how many passengers and how much cargo you load on the plane will change the figure. With the Freighter it will depend on how much cargo you load. The empty weight doesn't change.

Empty weight:           130875 Pounds
Payload:                     35650 Pounds
Fuel:                          75548 Pounds
Gross weight:            242073 Pounds
Max. gross weight:     256000 Pounds
Max. allowable fuel:     75549 Pounds

Adding the Empty weight and the Payload = ZFW 13875 + 35650 = 166525
Or
Gross weight minus Fuel = ZFW 242073 - 75548 = 166525
Zero fuel weight = 166525.
I would enter it as 166.6.  I round it UP. I never round down.

I use the first method. But that's my choice.

Hope this helps

Mark