Handout booklets like the Winchester you refer to almost always only list a maximum load. The rule of thumb is to reduce the data 10% and work up fom there, usually in 1 grain increments. This will work well when you have changed a component or two. When using exactly the same components, I only reduce by 5% and work up.
I rarely if ever have the same components, and I rarely if ever choose a midpoint load between that of several manuals. Choose one and work with it. That way you know where you are.
raynor is right, the rule of thumb for bullet seating depth is to be at least as deep as the bullet diameter. Most OAL are established to insure feeding through a magazine. The Handi doesn't have that restriction. Seating longer can reduce the maximum pressures, but I doubt it would be by enough to make a real difference. If you are seating a bullet longer than the listed OAL but still using a maximum listed charge, I would still consider the charge to be maximum, especially with a bottleneck case.
You're doing good whitedogone, keep it up!