You mentioned you had a problem with the plane climbing and that you need to add down trim.

Think about what raising the rear of the tail will do? This will give you MORE elevator and cause a bigger problem than what you already have. The front of the tail is what needs to come up to fly level.

If you are basing your incidence off of the flat portion in the center of the V-tail, that is the problem. It is not level with the incidence of the tail.