Would appreciate some advice as I can't find info elsewhere.
I have recently replaced the factory fitted stylus on my project debut 3 (stylus 5 I think) with an ortofon stylus 10. I am still using the stock ortofon cartridge (OM5E).
Do I need to adjust the downforce for the new stylus 10 (15mN) or the cartridge (17.5mN)?
If the arm was properly set up and balanced before you swapped stylus assemblies, there should be no further adjustments required. The cartridge should be tracking in the upper portion of its suggested tracking force range. Tracking too lightly will cause more groove damage than will tracking at the top of the VTF range. This is especially true in budget level arms.
I would suggest you consider a cartridge change when this stylus wears out. While the OM's are good cartridges for many entry level arms, the design of the cartridge dates back to the 1980's when many arms had less effective mass. A cartridge more suited to today's arm designs would be wise, if for no other reason than to improve tracking of off center or warped discs. If you like the Ortofon sound, stay with the more current Ortofon designs.