What to obsess over
They use stopwatches at McDonald's. They know, to the second, how long it should take to make a batch of fries. And they use spreadsheets, too, to whittle the price of each fry down by a hundredth of a cent if they can. They're big and it matters.
Small businesspeople often act like direct marketers. They pick a number and they obsess over it. In direct mail, of course, it's the open rate or the conversion rate. For a freelancer or small business person, it might be your bank balance or the growth in weekly sales.
I think for most businesses that want to grow, it's way too soon to act like a direct marketer and pick a single number to obsess about.
The reason is that these numbers demand that you start tweaking. You can tweak a website or tweak an accounts payable policy and make numbers go up, which is great, but it's not going to fundamentally change your business.
I'd have you obsess about things that are a lot more difficult to measure. Things like the level of joy or relief or gratitude your best customers feel. How much risk your team is willing to take with new product launches. How many people recommended you to a friend today...
What are you tracking? If you track concepts, your concepts are going to get better. If you track open rates or clickthrough, then your subject lines are going to get better. Up to you.