Wow, this is really being over complicated. Get an RTA, put the time marker in, slow to a speed that you are comfortable with (I personally will go to Green dot +10kts), enter that as a descent speed and if you can't make the RTA then 'JQXXX, unable to comply with RTA, request vectors/hold'. I am not going to spend my time trying to work out complicated equations to try and finesse an RTA that a good amount of the time is just unrealistic or puts the aircraft close to an undesirable state. Why bust your balls stressing to make an RTA, throw it back to ATC, if they want to give me an 8-10 minute delay and 100 nm to try and achieve it then they can take some responsibility for it.