FOBT payout percentages are calculated by the individual games on each of the betting terminals, however as players tend to switch between games, it is more beneficial to work out the long term return of the entire content of the betting terminal rather than break it down to the many slots and variations of roulette.
Individually, each of the FOBT games has an information screen, and for the most part they are fairly vague, however they do often tell you the long term theoretical return to player (RTP) and that the chosen game is random (mostly). You can expect slot machines to have an RTP of between 90%, up to 94% depending on the chosen stake, and standard roulette to have a long term average RTP of 97%.
The biggest problem with this apparent high return to the player is the speed at which the return can take effect. £100 will (on average) return £97, then 20 seconds later that £97 has dropped by another 3%. This process is repeated constantly, with the majority of players playing out their money to £0 within an hour.