Currently, Magnus acts as a bonus before policy cards/Goddess of the Harvest are applied.
So, if your base forest chop is 50, then Magnus adds +100% to that, for a chop of 100. So if you have Goddess of the Harvest, you get 100 faith. If you're running Limes, then the +100% is applied to that amount, meaning you get 200 hammers towards the walls. Hence, Magnus is OP.
Thanks for the info! Am I thinking correctly that overflow works with or without Magnus?