Our temps are about the same here, perhaps a bit warmer during the days, and I blanket clipped my mare last week, I think it was? (not cushings, clipped for convenience, she's sweaty pig when ridden otherwise)
I find I need to have a medium weight on her at night or I find her shivering in the morning. The sheet just didn't cut it overnight.
If you're hoping to clip and just use the sheet, I think it needs to be just a bit warmer.
I'd hold off until it's a bit warmer if pony only has a sheet. Last year we waited until it was steadily in the 60s during the day to clip my mare, but she did get chilly easily so we didn't want to jump the gun!
Sheep shears! I envy you. I wait till about May to do the old guy. I start fretting about it around now but I came home today to a frozen pool surface so I'm not going to do this for at least a month, say around tax time.