You'll probably be dealing with less flux applied - and less as you move up
the barrel from the bottom to top.  Flux activity & No-clean vs. RMA vs. OA
considerations are something to think about.   Appropriate sized/patterned
nozzles for your application that are kept clean are a requisite.  If the
spray fluxer sprays back and forth (perpendicular to the rails) then it will
be more sensitive to belt speed changes than one continuous head of foam - be
sure to verify consistent coverage.  Pressure is something easily monitored -
be careful about blowing out loose components if you're not clinching.  One
"tool" that I saw was an option for a removable clear flux window on the
WaveSolder Optimizer - you run it through with the wave off and then measure
the weight and subtract it from the "unfluxed" weight  + you can check for
even coverage at that point.  You can still check specific gravity but it
shouldn't change much if your drawing from a closed container - if you're
thinning you flux for some reason then you need to verify it anyway.

My 2 cents,
Mike Sewell