I would advise to use the same level for both freq bands.
The path loss of 10dB is due to free space propagation attenuation, which depends on the frequency.
Freq 1800 in the formula gives 6dB more loss than the freq 900.
There is more attenuation in the feeders.
The 1800 TRX are weaker than 900 TRX.
Then, 1800MHz is a bit more absorbed by obstacles (buildings, trees,…). This point is subject to discussion though.