Yes it would be less than 600 if there's no season change and no rain/snow falling on route 7 before the roamer arrives. I've gotten as low as 520 before in such a scenario. Mid 400s are probably a possibility too but definitely not lower than that.
It's determined when it flies off (or a little after that). Your PID frame isn't affected until you exit the house and yes you have to mash A to get consistent results. Here's a long post on roamer RNGing.
Idk, on the emulator it didn't rain in Spring or Autumn but I thought that was because I changed the date on my computer after hitting my seed. Perhaps there is actually a way to make it not rain in Spring/Autumn on the DS. I'll have to try it when I find the time.
And lol, timer0 trolling.
EDIT: You shouldn't be doing 0, 1, 2 etc advances. It should be 0, 5, 10 or something like that.
It's going to be nearly impossible to manipulate the PIDRNG in rain. It could be anywhere between 1500-1900. Some people have even gotten a little higher than 1900 (like 1950). It's just not feasible to try to manipulate the PIDRNG in rain on the DS. :/
Try a summer month.
EDIT: In Summer it's like 500-750. It could possibly be in the 400s if you do this. That's why summer is best.
Yo Nice Thread. Although you should add these two links because it would be useful for beginners trying to RNG. They are the encounter slots (and explain encounter slots in the faq when you get down to it) in pokemon black and white:
Sorry I don't want to be a bug and tell you things to improve. This is just something that would be greatly used without having to open the RNG reporter.
Btw, it's not actually supposed to be 2 parts. It's meant to be 1 post but it was too long for a private message so I had to break it up into two parts.
Add anything necessary, remove anything unnecessary.