It's probably best to just assume 125. Firstly, it is hard to get an accurate measure. You could use displacement methods --- take out a decoration and measure the changes in the height of the water level. But, you'd have to have the water level to be still, so you couldn't do ti while the filter is on.
However, just assuming 125 isn't going to be too bad. It isn't terrible to slightly over dose, if only because a lot of the medicines lose their efficacy very quickly. A lot of the ich medications work by oxidizing organic material, for example, and that means everything organic -- uneaten food, fish poo, dead or decaying leaves of plants, etc. If the medicine is oxidizing these other things, it isn't oxidizing and killing the ich. This almost can't be helped because there is a lot of organic material in a fishtank (it can be lowered a little by doing a good cleaning of the tank before medicating). Light and especially sunlight destroys the efficacy of many medications.
So, adding a little extra actually keeps the concentration at a therapeutic level a little longer. Now, the flip side is not to get too carried away, doubling the dose of medications isn't a good idea either. But, the 10% or so error from using 125 instead of the actual volume of water in there isn't a big enough error to worry about. There are other errors like, not accurately measuring the dose out, or not shaking the bottle to mix the medication up, that also enter into it that introduce just as much error as anything else.
