Email conversion rates for pre-launch

This may be a loaded question, but what do y'all think would be a good email sign-up rate for the number of unique visitors on a pre-launch website? #ask-ih

Also, are there any other metrics you prefer to use for these sorts of MVP testing? Any experience and advice from the blog/online content creation community would be hella appreciated! Asking for my new project coming soon :) but my experience in email marketing is limited as of now!