Losing lots of chlorine is what is expected to happen. You just keep adding more till the level stays the same overnight. It isn't critical to know the exact pool volume. You are moving along, even if it doesn't seem like it. It always takes a number of days of adding chlorine before the FC level remains at reasonable levels overnight.
When you are doing a baquacil conversion the chlorine goes away very quickly, so seeing low FC levels the next morning is to be expected. Once you are further along in the conversion, and chlorine is hanging around for several hours, it will be possible to get an estimate of the pool volume using water testing. But right now you will have already lost most of your chlorine before it has mixed into the pool and can be tested, so you can't draw any conclusions about the pool volume.