After exhausted all of the original 60 training points, how many more do I need?
Depends on the legendary. 120-180 training points are needed most of the time.
Total or additional?
Total.
You can feels safe dumping 6 or so super youth fruits into your legend and then slowly incrementing using both supers and regulars until maxed.
What legend is it?
Darkrider and Santa Penguini.
Yeah, usually around 130 TP in total to evolve a legend, although some newer legends would need more.