What I would do is figure out how much to add by adding juice to a small amount of beer and verify the result.
The plan:
1. Pour a glass of beer (6 oz), then measure out a specific amount of juice (1/2 oz).
2. Pour a glass of beer (6 oz), then measure out a specific amount of juice (1/4 oz).
3. Pour a glass of just beer (6 oz)
Taste. Is it too much juice? Not enough?
Did the addition of the juice make it too sweet? (ruin the dryness of the cream ale?)
Drink all three and compare.
When you have figured out the right amount scale up the ratio. See if it works.
