I haven't seen any formal 'calculators' for this type of thing. The size and type of soap are likely to cause any estimates to vary. However it should be relatively easy to calculate in your context. What I suggest is that for any public facilities you construct you station a handwashing monitor/observer by the station. This person can encourge people to wash their hands, track how many people do so and document how frequently soap and water needs to be replaced. If you do this for a week then you should be able to build up a decent picture of the operational factors that need to be in place to sustain facilities in the long term.