It doesn't work in the binary fashion you described. Low sun angles have two results: Cosine effect and greater atmospheric absorption. The cosine effect decreases the watts per square meter you receive, if your skin is not normal (90 degrees) to the incident sunlight. Greater atmospheric absorption means more UVB is absorbed by the atmosphere before it gets to you, but some still gets through, as anyone who has ever been sunburned in the winter can attest. Higher altitudes and snow on the ground both increase the amount of UVB you receive, even when the sun is low.