Hi Tanglefoot, I picked up a rule of thumb somewhere that's easy to remember (that's why I remember it

). Slowly raise the temperature by 5 degrees when you're at half the original gravity. Half and 5 seemed to stick, dunno why

And sorry to contradict the previous posters but I went from 13-18C and it came out fine...perhaps I was just lucky, who knows.
Anyway, just hold it there until the diacetyl has gone. To test that just warm up a sample and taste it, not sure if there's a more scientific method but it worked for me, if you can still taste a buttery note the yeast is still doing it's thing so keep holding it at that temp, even if it means holding it there until you ferment it out completely. When you're done, lager the sucker
Hope it all turns out OK for you

"Everybody has to believe in something, I believe I'll have another drink." - W.C. Fields