Thanks for the info Charles. The parts I sprayed just a week or so ago all turned out with a bit more gloss than I anticipated given the 10% of flattening agent I added. Seems likely that what you indicated was in play. I got lucky with these parts in this case. I think the level of gloss is just a touch less than without the flattening agent. And, I think that matches the original parts well. The steering shaft and driveshaft, sprayed more than a year ago, are definitely duller, and the mustache bar is duller still.
I think I will go with 10% again, let the solution sit for 10 minutes, and attempt to spray everything as I did last time. I weighed the individual parts of the paint solution (paint, hardener, reducer, and flattening agent) using a digital scale. So I should be able to reproduce my last effort (duly noting your info about not spraying too heavy). If I can replicate those results from a couple of weeks ago, I will be happy. I just don't want to end up with artificially glossy parts - in my opinion, that would be the result without any flattening agent.
Also, the next time I spray paint on these parts, it will include all of the rest of them. So, I won't be doing several batches. The only reason I did this batch separately is that I had already blasted and primed them ages ago. For the rest, all the blasting, priming, and painting is ahead of me.