To explain it more clearly I see two problems with the drive calculations:

(The ship is 1800 Dt with a 1200 Dt drop tank for a total volume of 3000 Dt and has a computer m/4 using 2 EP.)

1: EP produced is based on ship+tank, EP needed for agility is based on ship alone, in the drive section. The drop tank section is correct. Note that agility is 3 in the tank section!

2: When using standard LBB2 drives, potential is based on ship alone in the drive section, but the EP produced is based on ship+tank, hence the produced EP is incorrect. The tank section calculates the correct drive potentials, but presumably uses the incorrect EP produced to calculate an incorrect agility:

Note that with standard drives the ship+tanks and the ship alone leads to two different values for EP produced (and required for full agility):

Ship+tank: 3000 Dt × PP-4 × 1% = 120 EP.

Ship alone: 1800 Dt × PP-6 × 1% = 108 EP.

So the value calculated in the drive section should not be used in the tank section...