For me personally, I drop an assistance lift and reduce my main lift and assistance to 75% of my 3 rep max, aiming for 3 to 5 reps.
You can go either way with what you described above. Not black and white, so give both a go and see which one works for you.
Generally the rule of thumb I use is, my heaviest set's final rep should be fairly easy to get up. That is just me though...
And overall I should be leaving the gym feeling worked, but not exhausted.