A diacetyl rest isn't necessary at all if your fermentation procedure is sound.
If you pitch the proper amount of clean healthy yeast, and pitch cold...say you want your final ferm temp to be 50, pitch at 46 then slowly allow the beer to rise to 50. After 2-3 weeks keg, lager and youre done. This is the Narziss method.
When you need a diacetyl rest....if you pitch warm....say 62 and then lower your temp to 50, you will want to do a diacetyl rest for 3-5 days. between 62 and 68 is fine. Then you will want to SLOWLY lower the temp down to lagering temps.
Hope this helps!