ralle72 & AE
The e-e-l-l-n-n pattern is over is a 7 day work pattern, as duty is accomplished on all 7 days day. The part of the day left after the last N is classically called "a sleep day". Thus with 3 days off it's a 10 day pattern, hence must be plotted over a full 365 day period to ascertain/input leave/trng/public holidy requirements. Irrespective of actual leave taken, this must be plotted in the planning stage to see a realistic picture not just for manning, but for financial planning for the bean counters, i.e. how much overtime etc.
Also gives you ability to switch personnel from one team to another for quality assurance flexibility/adaptability. Creates good CRM.
With 27 you sure are in a creative position, a luxury not normally heard of these days, when I first started (long, long time ago) we had 5 to cover 24/365.