Joel Spolsky once wrote an adticle: https://www.joelonsoftware.com/2007/10/26/evidence-based-scheduling/

• The mythical perfect estimator, who exists only in your imagination, always gets every estimate exactly right. ...
• A typical bad estimator has velocities all over the map....
• Most estimators get the scale wrong but the relative estimates right. Everything takes longer than expected, because the estimate didn’t account for bug fixing, committee meetings, coffee breaks, and that crazy boss who interrupts all the time....

As estimators gain more experience, their estimating skills improve....

In the article he goes into how by knowing how "acurate" the guestimates are, that you can simulate and predict from their next guestimates the buffer needed.

Joel Spolsky once wrote: https://www.joelonsoftware.com/2007/10/26/evidence-based-scheduling/

• The mythical perfect estimator, who exists only in your imagination, always gets every estimate exactly right. ...
• A typical bad estimator has velocities all over the map....
• Most estimators get the scale wrong but the relative estimates right. Everything takes longer than expected, because the estimate didn’t account for bug fixing, committee meetings, coffee breaks, and that crazy boss who interrupts all the time....

As estimators gain more experience, their estimating skills improve....

In the article he goes into how by knowing how "acurate" the guestimates are, that you can simulate and predict from their next guestimates the buffer needed.