Look under Rickbeer's post for "How to Read a Hydrometer" Bonsai & Brew provided the equation. There are on-line calculators such as the one on "Screwy Brewer". Worth taking a time to read and mull over. BTW - Welcome to the Forum.
Welcome to the brewing business
So at 3.9% it has a way to go to be done fermenting - it should go close to 5.5% (claimed). Also in case you are not familiar with the hydrometer (pls excuse suggestion if you are), make sure you read the bottom of the meniscus. You also should test hydrometer calibration in plain water just in case it might be off.
I liked the Long Play IPA too, it is lighter by far than many commercial brews.
Live and learn. Until joining the forum and being a clueless newb, I didn't know that the high krausen generated that much heat diff over ambient. Lol, ignorance is bliss short-lived. So I'm two batches in and nothing to be done but correct for the upcoming third.
I'll definitely have to look at getting a small fridge or freezer with an inkbird before long.
However, my next batch the lbk will go in my cooler with an ice bottle and temp probe. Ambient is 68 in the upstairs, so shouldn't take too much to drop it 3-4 degrees.