I think it was documented in the "HST FAQ's". Basically, if you know your 15RM, you can calculate (i.e., guestimate) your 10RM and then your 5RM. The calulations are:
10RM = 15RM * 1.2
5RM = 10RM * 1.15
You can work with the formulas to estimate your maxes in either direction, depending on what your base starting point (RM) may be.
I've used this for my maxes and found that they are pretty close. I used the calcs to test my RM's and found that I could up the weight slightly on a few exercises, but the calcs were usually within 5 - 10 lbs. I err on the side of caution for the maxes, meaning that when the weight was just above a 5lb multiple, I'd round the weight down to the 5lb multiple. So, if bench came out to 127, for example, I'd round down to 125. If it was 128 lbs or 129 lbs, I'd go ahead and round up.
As for pull-ups, a lot of gyms have assisted pull-up machines with counter-weights to lighten the load. Otherwise, use a wide bar on a lat pulldown to get similar benefits of a pull up.
Hope that helps.