How do you find it?
Keep doing the lift with heavier and heavier weight till you cant lift it. I dont know if you are looking for a formula or something but since everyone is different its best just to figure it out.
Ive found this to be pretty close for most basic exercises
That one isn't too bad, but it's a bit off for me (lower than my true maxes). I've found this one to be nearly 100% accurate for me: