@2c1a2dac That's a good question - Trying to join the end result does not go well. I think joining the strips before turning it into filament is the best way, but I have not yet tried that.

In general, I rely on a filament sensor or simply attending to the printer. Alternatively, in the slicer, you can get an estimate of the weight of the filament used on a color swap, and space color swaps to use most of a line of filament. All techniques come with caveats.

As far as not drinking soda, we ask 

 @2c1a2dac for bottles from friends and family. 2L bottles give the best yield and generally have consistent thickness so that is what I ask for.

You are correct that those non-carbonated water bottles don't work well. The wall thickness is far too thin to make a filament. You would have to start with a strip so wide as not to make the effort worthwhile.