I think a lot depends on how good you are. What is your ROI like ? If you are a marginal winner at the level you play, say 5% Then I think 5 buy ins is a
reasonable amount. If you are crushing the level then 25 should be just fine.
The higher you go the more padding you will need. It can hurt bad if you move up a level and with out doing much wrong lose your first 5 or 6 sng's
More mentally than anything. So I would say take a shot at the next limit when you have like 30 buy ins, and then move back down a bit sharpish if you get off to a bad start.
I think Ferguson's guidelines are good, but a little thin for some players, and they are largely dependant on you being able to shift up and down quite a bit without it bothering you.