Is the FF (Fuel Flow) the fuel consumption index? I am creating a program to calculate the fuel needed to fly and I need to know the fuel cost of the plane. In the boeing 737-800 (PMDG), on the display that shows the plane's systems, there is the FF (Fuel Flow) field. I think that is the fuel consumption index.

However, using a stopwatch, the value there appears to be incorrect. Advancing the throttle to keep the FF at 5, about 10 pounds was spent in 8 seconds. I expected 10 pounds to be spent in 2 seconds (5 pounds * 2 seconds = 10 pounds).
My question is: does the FF indicate the plane's fuel consumption? If so, does it indicate pounds per second? If not, how can I know the fuel consumption?
The FF value can show either the fuel flow or the fuel used, depending on the position of a switch on the center forward panel. If it shows the fuel flow rate, then the value shown is in thousands of kilograms or pounds per hour per engine.


1 FUEL FLOW Switch (spring-loaded to RATE)
RATE – displays fuel flow to engine.
USED –

displays fuel used since last reset
after 10 seconds, display automatically reverts to fuel flow.

RESET –

resets fuel used to zero
displays fuel used for 1 second, decreases to zero, then displays fuel flow.

2 Fuel Flow (FF) Readout (digital)
[Option - kgs] Displayed (white) – fuel flow to engine with FUEL FLOW switch in RATE
position (kilograms per hour x 1000).
[Option - lbs] Displayed (white) – fuel flow to engine with FUEL FLOW switch in RATE
position (pounds per hour x 1000).
3 Fuel Used Readout (digital)
Displayed (white) – when FUEL FLOW switch moved to USED or RESET.

(Boeing 737 NG FCOM 7.11.13 - Engines, APU - Over/Under-Displays)