In one of the first pastry classes I took many years ago, the chef pulled the syrup off the heat at 235 every time and it worked. I think there’s a range of temps for this buttercream to work, and it could well depend on the batch size. For years, I pulled the sugar off at 248 and because the stovetop was just a bit of a walk to the Hobart in that particular kitchen, the temp would rise to 250 or 252 by the time I was pouring it in. The buttercream was quite firm. Now I’m pulling the syrup at 240 and it works beautifully as well and is a little softer but still works like a dream.
I think there’s a range at which this works; based on my experience, the minimum is 235 and the max is 252.
I use a probe thermometer (the type that is meant to be put into a roast while it is in the oven, and then the gauge is a timer/gauge combination that you can set to ring when it reaches the temp you set. I’ve gone through at least a dozen of these in the last 10 years - the probe wire is very heat sensitive and doesn’t like being over a gas flame! I’ve been lucky to find them at home goods stores, for about $15 and I don’t mind if they only last a year.