Stainless turns easily, just dont have the RPM too high and keep the feedrate up, it is similar for milling.
Setting boring bars up you can do it several ways.
You can take a cut of an internal diameter and measure.
You can, if you have enough travel, touch off the opposite side and enter a negative value.
You can use a piece of tool steel, or similar, held against the part and bring the tool out on X, you will easily feel as it touches.
Pic below of the last method although it is for the opposite hand of tool than I think you are using so is on the opposite side.