I use one to two gallons of lime water... which equals about 2 tablespoonful of food grade calcium hydroxide.
It should not take this much though to precipitate all of the fluoride... NOTE - I would not use this much at all if I were a beginner at this. I am on the high end of usage and have an over sized tub too; thus more water in my bath than a standard sized tub.
A teaspoonful of CH mixed thoroughly into a quart of water and added to the bath should work for most... I did see solid precipitation.... white flakes used to form on all occasions to one degree or another - which told me that the amounts of fluoride in the water were likely not consistent - however, once I got my aquasana shower filter (they say about 50% of the fluoride is removed)... the occurrence of flakes was dramatically reduced in the bath - and at times I see none... I never had the solid flakes analyzed.
As an aside... one can find food grade pickling lime (Mrs. Wages) in grocery stores that also cater to home canners... once canning season is over... about $1.60 a pound on sale... a pound will last most people a good long while depending upon bath frequency.
Having no other way to deal with the fluoride... this is the way I would go... or look into the milk of magnesia/nigari method... I bet it works too.
Just using one or the other though, like milk of magnesia (magnesium hydroxide) if you had it at home, would be better than nothing... a tablespoon or so would likely be all you would need.