It would be quite a bit.
This is a VERY unscientific observation, but one gleaned over many years, and that is the first 50 feet of our corn next to the forest is pretty well stunted due to the lack of light. I would say that it is 18 inches to a foot lower then the rest of the corn. Last year we planted our family garden in a new spot where ONE tree stood in the way of the westerly direction and we could not believe how stunted the growth was in the shadow, and we are only talking an hour or so per day.
But there are other factors too that you may not have considered such as the effects of PH on the soil the closer to the forest that you get. That can easily be compensated with extra lime (
compost), but it does occur. You can also get some nitrogen fixing issues with the forest robbing your soil of nitrogen, but again that can be fixed with extra fertilizer (compost), but it does occur.