My dos centavos:
I use Tempilaq to find the timing with excellent results.
Tempilaq goes inside the neck, not outside (just throwing this out there for those who don't know)
I clean my brass after annealing, so the tempilaq gets cleaned out. No problemo.
Use the tempilaq to get your intitial timing, and recheck it every so often. If your torch was pre-heated the timing should not change on you more than about half a second.
Outside temperature definitely makes a difference. Always find your timing for each batch.
Let your torch run a while so the nozzle can come up to temperature before starting to anneal. A cool nozzle will throw your time determination way off.
Make sure your brass has had time to stabilize in temperature before annealing as well.
Buy a cheap electronic metronome. It is far more precise than counting in your head, and you can watch your annealing while you listen to the cadence.
Use a nozzle that gives you a single inner flame that comes to a narrow point.
Keep the flame down, not full on, directed at the neck/shoulder junction and aimed more toward the neck. I go with a 3/4" to 1" inner flame, and just touch the tip of that flame to the brass. Watch that the length of the flame doesn't change on you.