Cherries have a more singular flavor, I would go lower in the same situations.
You would use less cherries than grapes? But as I understand, the well-regarded Krieks (3F, Oud Beersel) use 3-4 lb/gallon. This
article about Cantillon St. Lamvinus states that they mixed "a two-year-old lambic fermented with a blend of cabernet sauvignon, cabernet franc, and merlot grapes". The grape-to-lambic ratio seems to vary between 1.5-2 lb/gallon. That seems close to what you're recommending. But why should it be less quantity than cherries, if cherry flavor is more intense and the sugar content about the same?