Yeah, when I went in the house, I was dead-set on getting what I came for. Even if it'd take a week (it really only took three days), I wasn't leaving until I got it.
Unfortunately, I can't help much more with explaining the PIDRNG.
The first time I abused a roamer, it was more of a 'just to see if I could' thing. I had very consistent results, and as such was able to get my desired Nature pretty quickly, and even catch Thundurus in a good-looking ball. I didn't think to search for what PID Frame I started on, which was an awful mistake, but eh. All I remember was Bashful, Timid. Yeah.
Later when I went for Tornadus on Black, I couldn't get a good reading on my starting PID at all. I narrowed it down to about two Natures, but that doesn't help much.
(Not sure if it'll help anyone, but I didn't mash because it always got me wild results. I would watch the corner of the screen for the arrow and press 'a' when I saw it. Relies more on reflexes than mashing, but it was way more consistent for me.)
Basically you have to mash, or find some consistent way to get through the conversation, in order to get good results. It's mostly blind luck, really.
I remember hearing about someone else that found their starting PID, it was in the 400s, I believe. I would think it'd be in the 400s-600s if you're trying to find it.
Once it flies away, that's it. IVs, Nature, and Shininess are set.
Needless to say roamers are a complete pain... when I get around to playing White over, I'll dedicate myself to figuring these things out.