Keep in mind when you look at Westside percentages that the bands are going to add a considerable amount of resistance on the bottom of the lift.
BP Max- 300lb
50% of max is 150lb
bands add ~45lb at the bottom and ~90lb at the top
So the bottom of the lift is ~195lbs and the top is ~240lbs
Which means the bottom of the lift is ~65% of your max and the top is ~80% averaging out to ~72%
This isn't at all that far off from traditional PL programs that have you go heavy one day and stick to 70-85% for the lighter day. Really, the main thing that's different about Westside is the variety. It's just a bit on the extreme end for variety with Bulgarian or Sheiko depending on how you look at it on the other extreme.
And my experience doing it this way is that I actually do get more momentum to push through my sticking point. The bands really do help train me to keep pushing through the whole ROM rather than just popping the weight off my chest and relaxing the rest of the way so I don't hyperextend my elbows.
You're using the most amount of weight you can move quickly with a max effort for the volume you select. Some people will need more bar weight and some less. And don't forget that if you're feeling good, it's okay to work up to a heavy single, double, or triple. Just make sure it's a weight you will absolutely for sure be able to get and you can leave a couple or so reps in the tank too if you want. I actually found DE days harder than ME days.
As far as turning band tension into pounds, it actually gets pretty complicated and isn't just different from person to person, but different from rep to rep. The above math is just to understand the basic principle.
Having said all that unless you're benching with a shirt, your lockout issues are more likely a form issue. Raw lifters very uncommonly have this issue without it being related to form and usually miss at the bottom or middle third of the movement. Video!