I couldn't find better stat groupings, but Brown over his career:
30-39 yards 81%
40-49 yards 68%
50+ yards 58%
I would guess that with another 10 yards the % increase would have been about 10%.
I think the difference between 49 and 39 is more than 10%, but I could be wrong. I'd say a 39 yard FG in a dome is about 80% successful, whereas the 49-yarder is about 50%.
Given that his conversion rate for 50 yards or more has been 58%, I don't see how you could argue that from 49 yards it would only be 50%.Brown's success rate on kicks indoors over his career:
30-39 82%
40-49 82% (Prior to last night)
50+ yards 75%
The NFL average at 50 is around 50%
http://www.pro-football-reference.com/blog/?p=1716
I see Brown as 4/6 on 50+ yard FGs from indoors, but maybe he's 6/8 or whatever you saw to get him at 75%. Either way, that doesn't differ materially from the NFL average (a sample size of 6 or 8 FGs is not significant). Only the best FG kickers attempt 50+ yarders, so if anything you need to be a good kicker to even be included in the average of roughly 50%. Of course, a 50 yard FG is easier than your average 50+ yarder, which is probably around 53 or 54 yards.
Anyway, I feel that it's reasonable to say that the kick last night was a 50/50 proposition, or close to it.