Estimation is an art, not a science. It's always going to be a judgement call by the engineers tasked with giving them to management. Taking all of the factors from this article and beyond can and should go into making that judgement call.

I always tell my teams just skip the middlemen and think of estimates as time from the jump. It's just easier that way. As soon as an estimate leaves an engineer's mouth, it is eagerly translated into time by everyone else at the business. That is all anyone else cares about. Better said - that is all anyone else can understand. We humans all have a shared and unambiguous frame of reference for what 1 hour is, or what 1 day is. That isn't true of any other unit of software estimation. It doesn't matter that what one engineer can accomplish in 1 hour or 1 day is different from the next. The same is true no matter what you're measuring in. You can still use buffers with time. If you insist on not thinking of your labor in terms of hours spent, you can map time ranges to eg. points along the Fibonacci sequence. That is still a useful way to estimate because it is certainly true as software complexity goes up, the time spent on it will be growing non-linearly.

You can improve if you follow up the estimates. My team had several months when we were within +- 10% in the aggregate.