Hi Jack
ISIS automatically estimates the resolving power from lamp line widths for you and adds it to the fits header (SPE-RPOW) or if you want to manually measure it (for example on different lines across the field) you can produce a spectrum profile using the lamp image and measure them directly using the FWHM function.
In ISIS you can even measure the width of the lines (in pixels) directly off the image using the PSF function – useful for checking if you have best focus.
Cheers
Robin