I plugged up my downspout and filled my gutter full of water with a garden hose. The gutters began to overflow out the back of the gutter and not over the front lip. It's an older home, and my guess is that the fascia board is not perpendicular to the ground. If needed, can I shim out the back of the gutter at each hanger so that the water overflows the front. I haven't confirmed with a level, but if the gutter is tilting back toward the fascia, it is only a small amount. There is drip edge installed that goes down at least half way into the gutter.