Per Schroth, who IMO has done more testing on this than anyone else out there, acceptable range of belt angles for a 6 point:
(the downward facing shoulder straps not being acceptable because they plan load on the seat that the seat wasn't designed for. As a result, for every single car/seat they sell the quick fit for, they test the factory seat to make sure it can take the downward load applied by the belts... and they also worked with Cobra/tested the Nogaro to make sure it could take that load)
Their full instructions:
https://www.schroth.com/en/segments/...qg_2019-10.pdf