OK, I have been outta the game for 3 weeks with a strained forearm. What % of my previous should I start at on day 1? I definitely don't want to reinjure myself or hurt another muscle by starting too heavy.

how in the world did you strain a forearm?

Bad grip while doing a barbell curl, it's testy since I tore the muscle a few years ago. That was a torque injury and involved a dislocated radius and a lot of pain.

Blackout, probably not what you want to hear, but I dont think you can assign a percentage to it.
It's really gonna be more like start light and see what you can progress to especially with any gripping exercises. Can you get on some machines for say, a tricep push down, a straight arm pulldown (lats), bicep, so that you can do a decent amount of weight without worrying about the grip straining the forearm?

just start out with really light weight and you'll find that you progress very soon. lift more to get the forearm used lifting than to get a workout. in a few weeks you'll be back to normal.