You should count each direction as the one-way cost:
LHR-FRA-DXB = Zone 1 - Zone 5.
In C, that's
20,000 * 0.5 (one way) * 1.5 (C multiplier) = 15,000 + cash + taxes
DXB-MUC-LHR = Zone 5 - Zone 1.
In F, that's
20,000 * 0.5 (one way) * 2.5 (F multiplier) = 25,000 + cash + taxes
25,000 + 15,000 = 40,000 miles + cash + taxes
However, if you fly any leg in First on LH, you have to add a further 10,000 miles as a LH F surcharge.
So the routing you've described should really cost you 50,000 miles + cash + taxes.