I fully concur with the KH. Rain water has zero KH, so many of us add baking soda to raise the KH before high rain events. Roof runoff would concentrate the amount of rain water entering the pond, over that that falls directly. In addition, particularly after some dry time, a roof will have significant dust and pollution deposit on it over time that is then washed off with rain events, making it even worse. Getting some form of diversion of the rain water away from the pond will be a big help. I am not aware of a maximum KH and any KH above about 80 or 90 PPM will yield the same pH, usually slightly above 8, depending on aeration, which drives off carbon dioxide. I have run my ponds with KH of well over 200 with no harmful effects, that I am aware of.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard