My bottle of Bio-Safe says "do not overdose" and "recommended for water changes of 50% or more." I made a math error, and double dosed (based on the change volume not the tank volume) over a few weeks (say 10+ changes) and noticed my pH declining. Finding this puzzling, I emailed the company. The dechlorinator is an acid, so they thought it could contribute to a pH decline if overused, but pointed out that they thought my 0.6 pH decline (7.8 to 7.2) was not significant or could be test kit error, so would not give the product the credit for the pH change.
When asked why the bottle said for 50% changes or more, they replied that it could be used for less (of course!). But the Doctor quoted on the bottle only endorsed it for larger changes. I have heard of people who don't worry about chlorine with 20 % changes. My tap water reeks of chlorine, so I aerate it. Takes about 24 - 30 hours to stop smelling of it. I don't use the conditioner when I get to aerate the chlorine out of the water first.
so, I still don't know an answer to how much conditioner to use when pythoning right back into the tank! Using a holding tank to prep water sure has its advantages.