An indicative offer is exactly that, it indicates how much they will pay based upon the information they have received. If this is simply financial info, that is all it is based upon. You have a long way to go as they will undertake both legal Due diligence and commercial due diligence these processes will forensically analyse both financial aspects and legal aspects of your practice. If the financial information is based upon the previous years accounts and your sales levels are not maintained this may affect the final figure.
Additionally the final amount will depend upon the final completion balance sheet of your practice (Sole trader or ltd company) which will include the goodwill figure.
Most corporates will only reduce or chip at the final figure if things are not what they appeared, and will usually honour their offers.
The key thing is to get them to maximise the GW, bu maximising the desirability of your business.