Hi,
Regarding the original posters question, I would just charge a rent.
I am in a similar position that we have a hair salon which also has a beauty room that I rent out but also available is the building next door that I want to have as a tanning salon with 4 additional rooms to rent out, these for nail, teeth whitening, sports massage or perhaps something else ( suggestions welcome )
The room we do rent out already is 2.5m x 4.8m, its a good size, decorated well, bespoke cabinets for its purpose and a shower unit.
I did pay out and get an electric couch and all the customers feel that the room and herself portray a quality service.
I charge £175 per week, thats everything included like electric, water, tea coffee etc.
Its quite a bit to charge but the point I want to make is that there are people wanting to work hard, build up a good name for themselves and offer a quality service and experience and appreciate a good base to work from.
I feel that not doing a percentage/profit share and charging a rent attracts a more determined and committed individual rather than someone that wants to give it a go but has nothing to lose.
We have now had the beautician renting this room off us for 2 years, we did give some discount for 3 months to help her get started ( £125 p/w ) and since then she hasn't looked back, she is booked fully 5 days a week and earning very nicely.
she has paid us our rent on time every week ( important to set this from the beginning) and we in return have not/ and will not increase her rent for at least another year.