I want to purchase a roll of ND for general use on windows. What is the most common density? Can on generic density do most jobs, or do you commonly use a wide variety?
Generally, the brighter the exterior, the more ND you'll have to use to hit a given int/ext ratio. ND6 and ND9 are the ones I use most.
IF my interior lighting is balanced for daylight (HMI or 5600 fluos). If your lighting at 3200k, 85N6 or 85N9 gel will do the job and "warm up" the exterior to match your lighting. I have always used the ROSCO materials.
Unfortunately, depending on exterior conditions, one size does NOT fit all.
Hope this helps,
Ken