If it was under radar control, the controller would have (hopefully) be monitoring the spacing required. Which although requiring all the normal care, is a bit of a "no-brainer" using a track offset for a flight that is known to have a highly accurate nav system. In practice (or how I or my colleagues would do it) a numerical value a bit higher than the required spacing would be chosen, to allow a comfortable safety margin. Eg: you need 3 miles, you aim for 4. Or 5. Depends on workload/other distractions. If it shrinks a wee bit, doesn't matter.