The obvious thing to do is to ride until the main tank runs dry, then switch it over before you conk out. I have found that invariably the main tank runs dry mid corner - probably because the fuel swishes to one side. I'm not sure if this is due to a slight oversight in my fuel tank design (cbr250rr) or whether all bikes do this, but to me it represents a pretty big problem - losing power mid corner sucks! So the question is does this happen on most bikes? I generally switch to reserve when i'm getting close to empty at the expense of having to take a bit of a punt as to how many k's i'll get.