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.
Start by removing small sections of the limb. Preferably the sections that point down. You can always remove more but you can't put it back.
Trim a little the step back and look at it. If it looks like it needs more taken off then remove a little more. You can wait a few weeks between trimming to see how it's going to look and react. Then trim again if needed.
The overall objective, if you believe the limb has a high risk of failure due to lack of taper, is to increase taper by reducing end weight and length over time—- maybe pruning it at 5 year intervals 2 or 3 times.
I would focus more on reducing endweight— depends on the limb ( let’s say 10-14 in diameter and 30 ft long?): you might make between 2-5 cuts removing 1-2 in diameter branches but leaving about 2/3 to 1/2 of mass of the the forks or other branches out at the end; you might only reduce overall length a few feet.
A lot depends on the architecture of the limb; lots of forks makes this technique a good choice. You can also simply head it back, and then thin the suckers to create a branch with forks and taper over time— but there will be a period of time over which the suckers are prone to breakage, more so than branches you would leave after thinning the end of the limb.
This is also what you end up doing in repairing ice storm damage. I have pruned a Siberian Elm in my yard twice since it got hammered in 2012— nearly every limb broke or tore out at 4-10 in diameter. The whole crown is just about based on suckers. Looks pretty good now.