312 lines is the standard for PAL 288p. I’m surprised your computer can generate so many active lines, because the content would have been hidden by overscan on pretty much all displays. Does the cut of lines hide any content when actually inside a game/program, or just in a test scenario?
Regarding V.Active; the total of active, backporch, sync and frontporch cannot exceed the number of input lines. So make sure V.Active+backporch+sync is less than 312 (frontporch will adjust automatically).