I don't have much of a singing voice....
The MoD is required to make up the shortfall up to a ceiling - GBP 548 a day or thereabouts for mortals like me - and GBP 700 ish for the demi-gods in Harley Street. RTMC at Chilwell takes all your circumstances into account, mortgage, pension contributions included. Mind you, if you are self-employed rather than salaried the RAO does require audited accounts, statements from current clients etc.
This left the MoD paying me more than the general in charge of RC(S) last Spring, which shows just how cock-eyed our military world has become nowadays. (Then again, the AAC invited me on the all-expenses paid vacation, I didn't ask them.
)
There is no X factor, though. It would be far too amorphous to calculate.