OK - I hear about 30 pounds of pressure when tamping. Some tampers even have a "click" or other pressure mechanism set for 30 pounds.

However, I did some math:

A 49mm tamper covers 2.92 square inches. 30 pounds of pressure equals 10.3 pounds per square inch.

A 53mm tamper covers 3.42 square inches. 30 pounds of pressure equals 8.8 pounds per square inch.

A 58mm tamper covers 4.10 square inches. 30 pounds of pressure equals 7.3 pounds per square inch.

Looked at another way, to get 8.8 pounds of pressure with a 58mm tamper requires 36 pounds of force.

To get only 7.3 pounds of pressure with a 53mm tamper requires only 25 pounds of force.

So what's the rule? 30 pounds with a 58mm tamper, but only 25 with a 53mm? 30 pounds with a 53mm, but 36 pounds with a 58mm?

Or is the pounds of pressure irrelevant (within reason) so long as the tamp is consistent, and pull time is adjusted with grind fineness?