This question can't be answered in the way you want. Let's say you have a website that people can sign up for a free book on stamp collecting. If the only people that visit your site are stamp collectors, you are probably going to have a very high signup rate. If you get a lot of visitors that dislike stamp collecting, your signup rate is going to be very low (no matter how much you optimize your site).
In summary of the above info, whilst your signup rate can be improved by optimising your website, the quality of traffic you bring to your site will likely have a bigger impact. With that in mind, signup rates aren't really the best thing to measure in their own right (ie. 6% could be great or terrible). You really only measure signup rates to gauge improvement or reduction once you have your own baseline.
What you can measure early on is how much it costs you to bring a visitor to your site (advertising), what your signup rate is (now you can tell how much it costs per signup) and then if you can estimate how much a signup is worth to your business (lifetime value might be $1000), well now you know how much you can afford to pay to bring someone to your site to make a profit.
Above all else, only care about metrics that you are going to be able to act on. If you are going to effort to collect metrics that you won't be acting on, you are probably wasting your time.