That's far too much precision. Most of the time you'd be happy to say that it's 0.7, but not 0.8 or 0.6. The ethical committees that grant approval for research projects are becoming more aware of the need to have the right number of subjects in a study. They require you to document your estimation of the required sample size, and they will not grant approval for research projects with too few or too many subjects. Small samples are unethical, because you can't be specific enough about the size of the effect in the population.
Large samples are also unethical, because they represent a waste of resources.
You can sometimes justify a suboptimal sample size by arguing it's for a pilot study to determine reliability or validity, which in turn will allow you to estimate the sample size for a larger-scale study. A suboptimal sample size is also the starting point for sample size on the fly . But let's continue with the traditional approach and some formulae on the next page .