@Quadgirl, as you have seen here, there are a million different ways to do it. You can pre-bolus some, and then take the rest later. You can do an extended bolus if you are using a pump. You can calculate your current BG, add correction factors, and then factor in different food absorption rates. On and on it goes.
But really, no matter how many formulas you have, it comes down to experience. The more times you do it, the easier it becomes. You get better at it.
As a general rule, for things that are very sugary and fast-acting (not slower carbs), the most important thing is to have enough insulin ahead of time.
Why? Because sugar will come in a lot faster than insulin!
So give your insulin a “head start”. Because the cake will be faster than the insulin!
Try to be trending down when you start eating.
How much should you pre-bolus or what percentage should you pre-bolus? It depends on how safe you feel if you are low. For me, it does not really matter how low I get. I can still eat! As long as I have not lost consciousness, I will eat the cake.
It’s pretty much a survival instinct for me. So I just blast a bunch of insulin and then eat the cake. And I eat more later if I still need more sugar.
Check out this post, talking about survival mode. As long as I am not asleep, my body knows to eat. Not sure if you are that way, but for me, I don’t have a problem just taking all of the insulin up front!
BTW, this was not a planned thing. This was simply my instinct responding to a low. 