This forum is about dead. Great job by the moderator who helped kill it, and is proud of himself for "chasing" me off arboristsite. Three threads in the video forum have had in the last 4 plus months.
The answer to the question "how much can I safely trim off it is PLENTY for the tree's safety and unknown for your own safety. If it's only one limb, you can take it down to a short stub without doing much to the overall tree's health. Generally, to shorten a big limb up, you can take 6.8. or 10 feet off and that should reduce enough force on the limb to keep it from falling in a storm. Another way to look at it is making a 2-4" cut and leaving a nice lateral. There was a time when industry standards suggested that a limb not be cut back further than to a side limb, one third the diameter of the parent limb. Though many uninformed arborists still act according to that teaching, it is very antiquated. One thing to keep in mind is that a lower limb must get enough sunlight to be a win for the tree or the tree will shed the limb after letting it die. So that's a consideration. Sometimes when you shorten a limb up, you also need to shorten up the limbs above it to allow the light to reach the lower limb. Pictures and more details are required to give you any further guidance.