Per fangraphs the league wide BABIP is generally .300 in a given season, which correlates with how the teams have finished in the rankings of BABIP over the years, so that seems right.
You are adjusting the Braves BABIP down to at much below the MLB average as they are above it currently. A .290 BABIP would have rated 26th in the majors last season. Considering they would need to be a good bit below .290 to finish the season there --- eyeballing the math maybe something like .285-.288 you would be predicting them to have a bottom three BABIP the rest of the way, which really doesn't make a whole lot of sense considering all of the information about this incarnation of the Braves lineup suggests they are above average on ball in play in what looks about like a half season.
I don't agree with your assumptions.
We know that BABIP can be a skill. I don't see much to suggest that this is a bottom third offense until injuries kick in or trades. I would expect any cooling off would be offset by Inciarte, Swanson, Garcia heating up (two of which seems to be in progress).
MLB BABIP is 290 for 2017. That is the reason I adjusted it for that. If/when the league BABIP goes up to 300 like it usually does then either 1 or 2 things will happen. Either the Braves BABIP will go up to match what it going on league wide so they maintain their ranking. Or if they do stay at the 310 BABIP range then their overall offensive ranking will go down as the rest of the league improves.