My appaoach to priming powder is always with the thought of ignition speed in mind...every little thing contributes to ignition speed / lock time from polishing every moving part with a dremel & buffing compound, to using large vents, to razor sharp flints, and using the fastest prime available, and I do anything and everything I can to promote the fastest ignition possible.
On the subject of 4F being more easily affected by humidity than 3F, I took BirdDog6's advice a couple years ago and when hunting, I just dump the 4F prime every 45 minutes to an hour depending on the humidity, and replace it with fresh...the cost of 3grns of powder, refershed 3-4 times on a mornings hunt is only a penny and a half.
$9.00 ~ 7000 = .0012857/grn
Times a 3grn prime charge =.0038571
Times 3 refreshes = .0115713
So the original 3grn prime, plus 3 refreshes in a 4 hour hunt = a grand total of .0154284...that's one and a half cents worth of 4F per hunt, takes about 10 seconds using a small pan charger from my shirt pocket.
I've had the good fortune to drop a hammer on seven deer with flintlocks now and so far the 4F approach works so good I'd be afraid to change it just to be changing it