Generally, when working project includes face cutting, outer cutting, and inner cutting three ways and rough or heavy cutting, of course we will try to extend the clamping length, and calculate the suitable pressure.
We don’t need to consider above problems if we machine the work pieces of general diameter. But we must consider the wall thickness if we work inner and outer diameters (excessive pressure will cause deformation).
For example: the pressure of the gripping surface 15mm long (as large as will not cause any pinch) bore roughing of cut can be unilateral 4mm still will not feed 0.5mm/rev turning length up to 150mm into vibration.
There is no formula for clamping length and pressure, because too many factors to consider (like the work piece shape, cut strength and feed ....)
thickness unilateral 1mm which pressure is up to 1kg/cm square speed only, 800R.P.M. . the gripping surface should be above 3mm (or more).