So far it looks like 250 or 343 is your starting frame. If you're doing it with no weather on the route before Thundurus arrives, 250 is probably the right one. You can try both of them to see if you can get the nature you want but I'd start looking near 250.
Btw, I used the PID%6 method to narrow it down. You can read about it in the third post of the RNG help thread.