Q: How is the VCG adjustment that appears in RA /EXTRAFSM derived from FSM?

A: The VCG adjustment in RA /EXTRAFSM can be derived from numbers you can get via STATUS /FSM and STATUS /TRUEFSM but it takes some arithmetic. If you were using RA /FSM you could use STATUS WEIGHT /FSM which shows the derivation of the VCG adjustment. But the EXTRAFSM VCG adjustment is on the fixed-weight VCG and this is not a normal application of FSM. Here are some commands that you could add to your run file in order to document it in your report:

solve
status tank /fsm
st tank /truefsm
\\
\To find the fixed-weight VCG adjustment:\
\ 1. Subtract the true FSM from the formal FSM shown above.
\ 2. If the result is negative there is no adjustment.
\ 3. If the result is positive, divide it by the fixed weight, {FWEIGHT:1}
\ 4. This is the fixed-weight VCG adjustment that will be used
\ in the righting-arm calculations to represent additional
\ formal free surface moment which is not actually present
\ in this condition.

However this will actually give the value of the CG elevation, which is not quite the same as the VCG adjustment unless heel and trim are zero. For example, if there is much heel present at equilibrium there could be a TCG adjustment as well as a VCG adjustment since it goes perpendicular to the waterplane.