Welcome to toffee Hell
Toffee starts to separate at around 250 degrees. I have tried different butters, room temp or frozen, different sugar (beet or cane). The only conclusion I have come to is no conclusion at all! We have better luck in the big round-bottomed copper kettle doing 12# butter batches, than smaller batches on the regular burner. I think the copper inverts some of the sugar, or the size of the batch evens out any inconsistencies in the temps or ingredients. I like to start the batch slow, bring to a boil and gradually turn up the heat. If it starts to separate, pour in some hot water and stir like crazy. i haven't lost a batch in years, but you have to keep a close eye on it. A friend of mine who is in the toffee business always uses non-stick pots. He said that really helps on the separating. I always take it up to 290 (5000 feet). My husband stirs the big batches and he likes to get the butter and water almost to a boil before adding the sugar. I don't like to reduce the amount of water before adding the sugar. It just really doesn't seem to matter. Sorry I don't have more answers.