Conversion Rate can be anything: lead, newsletter sign-up, sale, contact form send.

So I suggest to beak down your site to the following levels (example figures but not far off from typical web-starter):

100% unique visitors 30% saw registration page 21% signed up for trial 30 days 19% logged in 11% did 3 key tasks (made project, setup 1 test, checked 1 report) 5% logged in 3x within first month 2% upgraded to paid service 1.5% is still with us after 1 year

So you have these metrics and then you monitor them day/week/month whatever you feel like. And every percentage has to increase otherwise non of you technical improvements, bug fixes, marketing effort or support is working.