But OK, there are various reasons of donk betting and it might be too narrow view to only consider that V bets here only with the top 3% of his range(set or 2 pairs). maybe he donk bets with more than that.
I put it in CREV assuming that his pre-flop range looks like this - because he didn't raise pre-flop but just completed.
and here is the plot that shows the dependency of hero's check-raise EV from the % of range(from the top) with which BB donk bets on this flop.
So, it seems that if BB bets with top 30% of his range and never folds in response to hero's x/r then this x/r is -EV decision. And you know how these 30% looks like? Any pair, any king-high, any ace-high suited. Can BB really donk bet with these hands? absolutely.
And this make sense on napkin as well. the max number of your outs is 14 : 8 outs to complete the straight and 6 outs to complete a pair. In reality though this number is less than 14 because:
1) V blocks some of your outs with Ace-high or King-high cards
2) if you hit a pair it still can lose to better pair.
so, the number of outs i would say is 10 which gives us 40% equity. it's not enough if BB always calls your x/r.