Comps (comparisons). Take valuation and annual / trailing sales of public and private companies in your market segment. Calculate the price to sales ratio (valuation divided by annual / trailing sales) for each company. If there are recent private company exits, or private equity buyouts in your segment, note the price to sales ratios at the time of the transaction. Average the ratios, and apply to your own sales numbers, and you have a valuation. Of course, this is not a perfect approach, but at least its based on data, versus subjective opinions.