By picking an time which you assume will overexpose a bit, if you're correct, then you've successfully eliminated all the longer exposure times. If you drop back a stop, 1/2 the exposure time, and it's over, drop back again, if it's under, then you've bracketed the possible exposure times. E.g. 30 is over and 15 is under, then you know the exposure range is somewhere between, and it's not too hard to eyeball it and get close to what you want at that point. I usually use a combination of this technique and test strips. I cut a sheet of paper into long and narrow and try to position them so they hit both the highlights and shadows. A few test strips and I can choose the right contrast filter and get the starting exposure. Then modify as above. But I'm not darkroom expert, so find a technique that works for you.
What this procedure reminds me of is a binary tree. Start with your possibilities, divide by two (i.e. this is underexposed, so that eliminates all the shorter exposure times), keep dividing by two, and you've quickly homed in on your exposure time.