I would look at valuing the business in an entirely different way. If you just offer 80% of the asking price, it is rewarding somebody for plucking a figure out of thin air. Remember that when buying/valuing any business, you are buying a stream of future incomes- regardless of whether it is a shop, a consultancy, a manufacturer etc.
There are a number of models, some more approprate than others. There will be a standardish multiple of nett profit for the industry. You need to check that the nett profit is accurate- is it inflated due to no maintenance of machinery etc. If so, you need to make an adjustment.
You could also look for changes that you could make to the business, which would affect the nett profit. For example, if you could see that you could make efficiency savings, it is worth looking at how this could effect your valuation. Likewise, if there are streams to the business that you do not want, then value those appropriately.
Are there any synergies that you could bring from another business? If you are expanding, this could help you to get a valuation closer to the asking price, compared with somebody who has no synergies. For example, could the first business be ran from the same premises as the new one, thereby saving rent.
An alternative approach is to look at the assets of the business, and adjust for those which you do not want, for example has the MD got a sportscar sat on the books? This will give a much lower valuation, but gives you an idea that goodwill has been valued at, and another negotiating point.
The final sale price is likely to give a value somewhere between the asking price, and the figures you calculate. You would hope that the seller of the business has done similar exercises in order to value the company, so you can have a sensible discussion around the numbers. To return to my original point, you are buying a stream of future incomes. Taking away any emotions, there is a price at which this is attractive, and above which it is not. You should be able to be reasonably open with a seller about this- and shows how different businesses can make widly different offers for acquisitions, some of which dont seem to pay for the other bidders.
I hope that this helps
Mark