Yes, agree with Nate to an extent that conditioner dosing need not be an exact science, its a rough thing, but...
When dosing to condition the water in a bucket that will be added, you dose per the volume of water in the bucket. When dosing to the whole tank, as when using a Python or other hose setup, you dose to the whole tank volume, regardless of the percentage of water added.
Tanks that are cycling and tanks that are mature do not share the same needs for conditioning necessarily. The fragile, undersized bacterial colonies during cycling do not need to damaged by the kind of over-chlorination that water authorities often practice unannounced in their pipe systems. Thus, first the use of a good conditioner and secondly the slight over-dosing of it are a good thing for risk reduction.
A good dosing is 1.5x or 2x but not more than 2x of the amount instructed on the conditioner bottle. Fully dosing will take care of normal governmental guidelines for chlorination and chloramination. Overdosing up to 2x will take care of those unexpected incidences where water authorities determine they have a "bacterial problem" in your geographic district and overdose the chlorine products. Not going over 2x dosing of conditioner is to keep the conditioner from having a negative effect on N-Bac growth during the cycle, which was another of the things we learned from Hovanec's work.
My own rule of thumb for beginners that its a reasonable precaution to follow this sort of dechlor habit for about the first year of a tank. After that point the practice can be reexamined for possible money savings or overall necessity. There are many threads on TFF that have discussed the pros and cons in more detail. My reading here at TFF also has biased me towards liking Seachem Prime as a conditioner for beginners (possibly also Amquel+, which may be only US) not only because it is so concentrated but also for the quality of its added features and the positive feedback of members.
After a tank is mature (after 1 year), some members report they can find concentrated pond conditioners that may cost less per treatment than Prime. Depending on species and value of the stock, some members also choose not to condition water at all in large tanks when perhaps 15% or smaller water changes are made. All of that gets into the gray area of personal finances, perceived risk and perceived value, so its each to his own.
When using capfuls to measure conditioner, threadlines can be a useful volume indicator. You can use a marked syringe or dropper to determine roughly what various threadlines in the bottle cap would give you and then use them as a quick rough measure on a week to week basis.
~~waterdrop~~
