This may be blindingly obvious to everyone else but I only just figured it out and it seemed useful.
When I am ill and the illness is driving up my blood glucose, like most people I use a Temp Basal Rate to try to control it. I tend to do a correction dose over a 1hr or 2hr period and then use that correction dose to figure out how much to put the TBR up by. So let's say I need a correction dose of 1u over 2 hrs, that's 0.5u/hr, so I will put my TBR up enough for an extra 0.5u/hr. So far so good.
The problem that often comes up is that sometimes normalising blood sugar will control the infection enough so that it stops driving up blood sugar. So if I leave the TBR on for 12 hours, I may go hypo. Conversely if I leave it on only for 4 hours (the default on the Vibe), and since (doh!) the Vibe does not alarm when a TBR finishes, I could equally end up going high again.
The solution my small brain has come up with is this. Set the TBR for 2 hours, and set the post-bolus alarm for 2 hours (I normally have it set to this anyway so I can do a +2 hr post bolus test). Then after 2 hours the TBR stops, the bolus alarm goes off, I can check my BG and see if I need to continue the TBR, reduce it, turn it off, or maybe increase it. "Simples", as they say.