For correctness, 1.75mm is the nominal diameter of the filament. 0.875mm is the radius of the filament. (Surface area then is about 2.405mm^2)

However, what you've calculated there is the amount of pressure that there will be in the nozzle if the nozzle is blocked to the point where it can no longer extrude. Any pressure above that point will be lost when the plastic flows out of the nozzle. However, if what you're interested in is the maximum that the system can exert, then in that case, you take the rated motor torque, (Nm) multiply by the radius of your extruder and any gear reduction that your extruder has (This ignored friction losses, but...) and you will get N of force, which you then divide by 2.405mm^2 in order to get pressure (N/mm^2) If your filament does not flow at this pressure, your extruder will stall.

Actual pressure in the melt chamber will be dependent on the viscosity of the melted filament, as to what makes it flow through the surface area of the nozzle, so you will have lower pressure in the melt chamber with a larger nozzle than you would with a smaller one, and greater pressure at lower temperature than at higher temperature.

**MBot3D Printer**
MakerBot clone Kit from Amazon

Added heated bed.

**Leadscrew self-built printer** (in progress)

Duet Wifi, Precision Piezo parts