A good question.

There's going to be a lot of variation in current vs pressure because there are 3 energy form changes (electric to mechanical to fluid flow) and many outside influences.

Starting with electrical- is the operating voltage 115V? Or is it 121V or only 109V because someone is running a toaster simultaneously on the same circuit? All those will effect current and yet have nothing to do with pressure.

Is the motor warmed up or in a cold room? The shaft and pump seals, how does temperature, mineral buildup etc effect them? How often does the operator clean the machine? Does the grind of the coffee or make of filter influence back pressure or temperature or something else? All this, and yet even if you figure all that out it will change as the parts wear over time.

So what you will run in to is that there are so many things that effect current that you will never be able to entirely trust it as a measurement for pressure.

So the best possible solution is the simplest - avoid all the guessing and go right to the thing you want to know. It also allows you to build a check into your system if you want - of actual known pressure.

That can be helpful for safeties, interlocks, warnings etc.