One key metric is the ratio of customer acquisition costs to total subscription revenue. As there is typically a lot of churn, acquisition costs can easily outpace operation costs. It typically takes patient investors while you spend a lot of marketing money to build a subscription base. But after this starts to pay, you can't take your foot off the gas. If you have to spend a dollar to make a dollar, even if your total subscriber base is growing,you have a big problem.Salesforce is a case in point.