The lower the setting, the longer it will take as well as the less you will get back from the process. For example, with a setting of 75, you will combine 4 tapes onto 1 (25% full on each, send off 1 and get 4 back) and a setting of 50 you will combine 2 tapes onto 1 (50% full on each, send off 1 and get 2 back). Once you drop below 50% you will get less than 2 tapes back for each tape sent off. For example, if you have it set to 25%(75% full), you will send off 1 1/2 tapes to get back 2 tapes so you have to send off 2 tapes to get back 2 tapes. Since one of those tapes is only 1/2 full, we can say you'd have to send off 3 tapes to get 4 tapes back so that only full tapes are sent off.
I find that the larger the tape (3592, LTO4, etc) the higher the reclamation threshold just because it takes too much time at a lower setting. I keep mine at 80.
-Aaron