Proper ALC level depends on the rig.  Some want zero...some want a bit of ALC in order to get maximum (not full) power.
You have to know your ALC's behavior.  One way to tell is pretty easy.
Set your rig to 50W.   Adjust the WSJTX power slider to -3dB.  You SHOULD get ~25W out (-3dB will be half power).If you do not get 25W out then your power settings in your transmit audio chain are not correct.  Mind you I have seen a few rigs that do no operate in a linear manner but the vast majority do behave correctly.
Now start turning up the power slider in WSJTX to get 40W (you can just click the power slider and use the up-cursor key).
With the rig at 50W and transmitting 40W check your ALC meter.  Whatever it says at the point is what you need (Elecraft being the exception).  I've not seen a rig yet that can't do 80% power with a clean signal.  I've seen a lot of rigs where ALC starts coming in above 90% and it's best to avoid that .  Mind you it is NOT ALC which causes the problem -- it's just the canary-in-the-cave.   Mind you 90% power is 90W out of 100W which is a small enough difference of 1dB to not matter.
I have a paper on my QRZ page that goes into more detail .  Search for FT8Noise on https://www.qrz.com/db/w9mdb
Mike W9MDB