Great news! No ideal rule for power valves, that is a good starting point but it depends on how lean your main jets are and how much vac advance you run too. Adding more main jet or more vac advance, you can usually delay the power valve ie use a smaller number.
But, that's a detail - if it likes a 10.5, give it a 10.5.
At 90% throttle the pump squirter is pretty much out of the picture, so now I would start doing the paper clip test - Clip on, rev to 3000, then back off, pop hood & see if the clip moved any. Snug the clip up again against the secondary housing. Try again, go up to 3500. Etc, then 4000, 4500, 5000 if you want.
This way you can see when the secondary STARTS to open, then when it is FULLY open (if ever), then worry about the air/fuel mix.
Easiest way to richen it unless you have a bunch of secondary plates around is richen the primary jets. Really have to clock it somehow since seat of pants can fool you - usually richer feels faster because of the initial kick, but at some point richer becomes slower.
Then you can put in a richer secondary plate, and put your lean primary jets back in for mileage and to avoid carbon buildup in traffic.
Sure, you could disconnect the gas line back at the tank and blow forward, that's what I'd do. Pump test under load would still be good to make sure the whole system will feed under a long pull at full throttle. Should be fine at your level but that is a good check.
