There's always been a lot of debate on this. I'd think that no more than half-a-cup of puree or 1oz at MOST of an extract (possibly even .5oz depending on potency...)
I'd do a trick like splitting your batch in half once racking to secondary and use different amounts in each, or puree in one, extract in the other...
I have to wonder if the cherry would get lost in the bitterness, though. I'm getting these values:
OG 1.054, FG 1.011
Color 50.8 SRM
I'd think you'd actually need a good amount of cherry to be able to even detect the "hint".
(I'm enjoying BeerSmith 2 btw - can you tell?)