since 145 000 - 90 000 - 30 000 + 800 is in fact 25 8000 this should be right.
You have 30 000 fuel onboard when going off-block, burn 800 when taxiing and can take a load of 25 800 not violating the MTOM.
Just consider the MTOM being performance limited, e.g. by field-length. In this case a possible Max. Ramp Weight will be way higher than what's possible for you to take off-block and does not matter.