Sources are available from the British Franchise Association. You're in business to make money and there's no point choosing a growth strategy which doesn't maximise your profit potential. Some franchisors would be more profitable if they owned their own outlets themselves. On the other hand they would never have grown to a fifty or two hundred unit chain without franchising. In franchising you have to address these questions both from your point of view as the franchisor, and from the point of view of your franchisees.
Profit comes later from the on-going charges to franchisees based on their trading success.
In Europe the approach is to keep initial fees to franchisees as low as possible to maximise their chances of a successful business entry, and then to make sure that franchisees can see a value for money return on the continuing fees they pay. Continuing fees calculated as a percentage on turnover are preferable but some product distribution franchises inevitably rely on a mark up on goods supplied. The financial aspects of franchised businesses are just as much a specialist area as the legal. The British Franchise Association can steer you in the right direction. Potential franchisees will want to know what business they can expect to do and how profitable it can be.