Check the other thread about BW film
https://www.pentaxforums.com/forums/pentax-film-slr-discussion/45229-b-w-film-thread-2.html
If you are going to send the film out for processing you need to match your speed to the lab development cycle.
Labs run cycles of say 6, 10, and 15 minutes but no in-betweens
So decide on 1 or 2 films and then meter the light in an averag scene (with whites and blacks)
To do that select an "average scene" (one with whites, blacks and all the in-betweens) and then meter the light for box speed.
Expose -2,-1,0,+1,+2 stops from what the metering said, then a blank and then another scene in the same way.
After you send the film you will receive the scans/prints. Check for shadow detail and for whites that are not "bloomed" and select the best of the exposures. It maybe an inbetween as well, even though it is genrally safer to overexpose by 1/2 stops.
In that way the next time you load this film you will set the ISO accordingly. If it was 400 then -2=100, -1=200 [-1/2=320], 0=400, [+1/2=500], +1=800, +2=1600