Your were 900 feet AGL above ground level and you know that by the field elevation of a the closest airport?
You had thin margins to to begin with, it wasn't the best situation that you put your self into if you are worried about 100 feet.
If you are thinking about doing the NASA form for immunity, I'd say don't even worry about it, I am almost positive that the cops won't be at the door in the morning.
There are lots of factors, altimeter error, pressure error and you probably really don't know the exact elevation of the ground you were shooting.
Yes, I did go by the elevation of the closest airport. I'll be honest, I don't think filing one will help safety or other pilots in any way. I should have tried harder to get a Class B clearance, and when I could, just scrapped the video shoot for the guy, and tried it another time. I am kind of upset with myself that I didn't do that, but there is nothing I can do about that now, other than learn from my mistake. If there was any true mistake here, it was a lapse in judgement on my part. I understand what you are saying about the errors involved, not to mention, as I said above, this was not the congested area of the city I was flying over (by city, I just mean the borders of the city). It was right on the edge of a yellow area on a sectional, but it isn't "the congested area of a city" so reading the exact wording of 91.119, I would say I actually only needed to be 500' AGL. The city I was over has a huge mall with houses surrounding this mall, and I would call that the "congested area of a city" and straight line distance, I was close to 2 miles from this mall.
I guess I just wanted advise, and this place is great for that. I was tagged up on radar during the video, because I was talking to approach. Like I said, safety was in no way compromised. The only thing compromised was my judgement haha. Due to that, I don't think a NASA form would do anything, other than immunity for me, but at the same time, as you said, I may have actually been 1000'AGL due to errors, and the fact that I don't know the exact elevation there, plus the fact, by the wording of 91.119, I think I actually only had to be 500' AGL anyway.