First off, you are not getting paid enough for the MSK work, let alone the administration and organisational role.
The main question I would like to know the answer to is how much do you charge for an inital consultation and how much for a follow up treatment. From there I would suggest thinking of a percentage of that figure to work out your hourly wage. Figures I have been quoted by colleagues have been around the 40-50% mark for a basic member of staff. However with this extended role you are performing and possibly developing further that figure may need to be a little higher.
The way I always approach pay negotiations is to look at how much money I make the company either directly through physio treatment or in savings by them not requiring a receptionist or admin staff. This is very difficult for a boss to deny when you have cold hard figures backing up your argument.