You only need a small drop (mine is too much really).
What sort of temps to you keep your flat. I only heat to 19.5C/67F morning and evening and if I'm home in the day, which seems comfortable to me though lots of people I talk to on OB seem to think sounds cold, while my family think it's on the warm side... can't win
At night and in the middle of the day when I'm out it's on a lower stat set to 15C/59F, however it's only in the really cold weather in winter that it's likely to drop that far as the house is well insulated.
I find I don't need much heat at night, wrapped up in the duvet I get too warm if the temps don't drop. That night drop is enough for the phals to spike.
From reading other's comments here Phals really need/like warmer conditions than I give them. The drop to initiate spiking is a relative thing, they need to drop a few degrees, rather than needing a particular low temp. So if you start warmer you don't need to go as low.