45 min additions serve no purpose. Anything under 20 won't add much if any flavor (and zero aroma), just bitterness, but be less efficient at bittering than a 60 min addition. 60 mins for bittering. 20 mins for flavor (some like 15), 5 mins for aroma (or at flameout.)
Personally for Porters I either just do a 60 min addition and that's it (not caring about any hop aroma or flavor), or I may do a small 20 min addition to get just a tad bit of hop flavor. IPAs are a whole 'nuther story. For those I do a small 60 minute addition, then slam it with hops from 20 minutes down to flameout, then dryhop.