Great question. Hope you're having a good weekend too.
First, what business are you in? I think it's hard to say given whatever industry you're in. It could vary substantially (i.e. 10% re-order in one industry could be amazing in one industry, and horrible in another).
One way we used to think of it in the investment world was to evaluate a very simple formula:
CAC vs LTV. (CAC = Customer Acquisition Cost and LTV = Lifetime value for others who are reading).
In other words, how much does it cost you to get a customer at scale versus the value of that customer/user over their lifetime. Sometimes you plan on having only one interaction with a customer (say a wedding band retailer), or sometimes many - say with SaaS products. I guess my point is if you keep going back to CAC vs LTV, then it doesn't really matter. For instance one interaction at $100 of profit is the same as 10 interactions at $10. If you know your LTV is 2x your CAC, than you have a very good business.
I know it's hard to get a good idea on those numbers when you're early. But you can blend assumptions and real data. I.e. you know a customers first purchase = $10 profit, but you don't know how many purchases they'll make... you can fill in assumptions to guesstimate that amount and then compare it with how much it cost you to get them. If you're using totally free sources of customer acquisition (social media, PR), then try a few "test" buys using straight advertising... say $500 of Adwords to get an idea of how much it would cost you to simply buy that customer. That will give you a rough range of CAC.
Hope that is helpful! Love to tlak it out more if you want.
Also, if anyone is interested, I'm looking for alpha testers for a new design-your-own jewelry app. Let me know at email@example.com