BOAC - I can't fathom why you have to enter an estimated TOM either, but in this version of EFB you do (it has no 'calculate MTOM' option). As for fiddling with the thrust settings, the EFB defaults to optimum and will calculate the minimum thrust setting for each departure so to calculate the RTOM you need to manually select the highest thrust setting, in this case 26K then when you select calculate the EFB will display the suggested derate and assumed temp for the entered TOM but it will display the RTOM for the selected options (ie. engine derate, imp climb, bleeds off etc etc ).
The software is far from perfect and is extremely basic but the answer to the original posters question is exactly as I stated (assuming I've guessed the correct airline and therefore EFB of course!).