You should always know how much you're "worth" in the market at the moment - that is, given at most a couple months of job search, what is the maximum offer you're likely to get. [1] So, just ask the company for that amount - or a little more if you think the job might suck, a little less if you think it may be cool etc.

[1] In order to have this info, you should either interview semi-regularly or at least have a network of colleagues in similar roles who are sharing market info with you.