I use the newgrounds ratings system to rate all of my movies and games too.
I personally use entirely different sey of standards from your own.
The way you assign ratings is based on the explictness and offensive material regardless of the duration that it has been on the screen.
I don't factor how offensive the material by a first standard but instead focus on timing first.
after the amount of time has been measured.
I'll factor things such as explictness as secondary standard.
I highly recommend some viewer caution on even the lower rated movies and games as well.
Violence is not rated based on any blood or gore by it's first standard. the rating for violence is rated on the amount of time that you see it.
gory violence and santantized violence is almost treated the same by own rating system systems.
nudity like the rest of my system is graded on the level timing as well.
an A rating according to my standards might not necessarily mean that it is sexually explict or pornographic.
a lot of older A rated submissions were not sexually explict.
I never understood why the other catogories such as violence, language and subject matter didn't also factor into an A rating in many submissions.
You sometimes see M rated submissions that are more explict than a lot of A rated submissions because it had less nudity.
The one exception where I change the standards for rating of timing instead of explictness would have to be language.
cursing as spoken dialouge or text are two different things.
adult themes can cover a very broad range of things and is based on the subtext and details.
I do think the mesurements for the newgrounds rating can be expanded upon but it's not a perfect system.
No rating system is perfect at all.
They have all of these different standards and set of rules.
It's interesting to leatn how other artists self-rate their stuff.