Hi,
'At this dilution, none of the sera from patients with either uveitis or genetic-based retinal degeneration "were" reactive and only 5.6% of the normal sera were reactive.'
'None' can be followed by either a singular or plural verb. 'Were' sounds fine.
I'm more inclined to ponder '5.6% of the . . .sera were'.
'5 of the sera were' is certainly correct, but '5.6% were . . ' sounds a little bit odd. I guess it's OK, because the substantive subject is 'sera'.
Best wishes, Clive