How can I calculate the fuel consumption of an Airbus 320-200 at various loads?
If you need precise values, you need to provide much more detail. Fuel consumption is affected by many parameters; see this answer for more detail.

For a precise estimate it will be best to employ a simulation software; see the answers to this question for more detail.

If you are just interested in a general estimate, the venerable Breguet equation will already give good results. First we need to know how much difference the 20% in load factor makes: The average mass of a passenger plus baggage for short-range trips is 100 kg, so the zero-fuel mass would be OEW + 18 t = 60.6 t in the 100% case and 3.6 tons less in the 80% case. Allow for 3 t of fuel reserves and plug that into the Breguet equation, using a fuel burn of $b_f$ = 0.000018 kg/Ns and a speed of Mach 0.78, which equates to $v$ = 262 m/s in 11.000 m altitude:
$$m_1 = m_2 \cdot e^{\frac{R\cdot g\cdot b_f}{v\cdot L/D}}$$
The L/D should be 18 and for the range we use R = 2000.000 m. $m_1$ is take-off mass and $m_2$ landing mass. This gives $m_1$ = 68.54 t for the 100% case and $m_1$ = 64.66 t for the 80% case. To be realistic, you would need to include different fuel flow during climb and descent, but the real value shouldn't be too far from the 5 rsp. 4.7 tons we get from the Breguet equation.

Note that when you repeat the calculation for 5,700 km range, the take-off mass comes out at 78.7 t for the 100% load factor, which is almost exactly the MTOW of the A-320. With 2.3 t of reserves, the Breguet equation gives you exactly the specified performance of the A-320-200. This should give you some confidence in those results.