Okay, great! That’s the answer I was looking for! (And the answer I already knew. 
)
So now it’s easy for you. 3.7 units was not enough. 8 units would be too much. So you whittle away at the difference until you figure out the right amount.
Add a little bit to 3.7 and see how it works.
But understand this - You don’t know how much adrenaline your body released from that swim. You don’t know how much that adrenaline caused your liver to release glucose. You don’t know what percentage your liver glycogen storage was at, like if it was full or half or what it was. You don’t know how much cortisol your body released from the swim, and how that cortisol affected your insulin resistance.
You can try all you want with formulas, but they don’t work because there is too much you don’t know.
So instead of formulas, use… The Force!
One thing I can tell you. Whatever insulin you need now after your swim, in 3 months you will need much less.