My guess...and this is a guess.....is that if you are entitled to 650BD (married) you will get that OR if your base rent is above this amount you will pay the difference.
All will get the utilities paid to their payslip and this complicated mess with the taxes will disappear. You get 650 regardless of how you structure the contract.
If you are currently paying LESS than 650 you will be credited the difference on your pay.
That would make sense anyways and, at the same time, rid housing of their silly formulas regarding taxes and furnished or not etc.