Given a viscosity is it possible to calculate required pressure for desired output?
The answer hews close to the famous cliche "In theory, theory and practice are the same. In practice, they are not."   That is to say,  yes, there is a nonlinear but repeatable relationship between viscosity and pressure and feed rate.  However, it's strongly dependent on temperature, and further the apparent viscosity is a function of the pressure (think oobleck).    

In the end, it depends mostly on whether a couple percent variation matters to the final product.  (and if it did, nearly all hobbyist extrusion printers would fail)