I just double it. I give them a half scoop of grain in the summer, because they are outside most of the time. The only time I feed them more in the summer is if they need to gain weight. But in the winter I feed them a full scoop, since there is nothing in the pasture to eat, except the hay you put out.
My horses don't work very hard. They get the same supplements, but their hay intake tends to increase. I always have free choice hay out for them and I monitor how much they eat -- if a lot is left over, I adjust and if it's all gone I add more.
In the summer my horses are on pasture w/ extra minerals available. In the winter they get hay, minerals, a vitamin supplement, and some low-protein concentrated feed (how much based on age/breed of horse).