Ask HN: Selling to a Co-Founder

We started our software consultancy business four years ago, and there were three founders. One of them has now offered to buy the shareholding of myself and the third founder.

My question is: does anyone have any idea of how to value those shares? The company has no physical assets, so my initial thinking is that it would be based on potential earnings over the next x months or years, also maybe taking into account current turnover.