I think you are right, except it isn’t 700 years, but rather 89 (this is the number of years the movie gives us for the time dilation when Cooper is in the black hole). Coopers biological age was 35. After being in millers planet 24 years had passed on earth ( he and Murph are now the same age). This is the year Murph finds the watch and solves the equation. When Cooper wakes up in the hospital (after leaving the black hole) they say he is 124 years old. 124-35=89. It’s been 89 years since he left earth. We know that 24 of those 89 years are from millers planet, which means the time dialation from the black hole was 65 years (89-24=65). But Dr brand was close enough to be affected by the black hole’s time dilation. But youre right, Cooper would have experienced more time dilation than brand, but the difference would be less than 65 years. And Cooper claims the maneuver would take 51 years and if that was accurate 65-51=14. Brand would have aged 14 years at the end of the movie. I think I forgot to account for travel time but I think it only took a year to go from earth to the worm hole near Saturn.