Crystal-less RC calibration error tolerance

[BarsMonster]

Hi there, I just managed to run my first V-USB project on Mega48 without crystal, and did small investigation on working OSCCAL range:
Autodetected OSCCAL is 0xF1-0xF2 for both 3.3 and 5v power supply.
Device still works in 0xEF-0xF6 range, so it is not as fragile as I've thought.