Hello yoanlesouder,
After taking a quick look at your question I made up an example that could help you to get pointed in the correct direction. I hope that this information helps.
Example:
In the example below I get the hit location and I then figure out a point that is 100 units from the hit location towards that actor that fired the original line trace. The float value mentioned in the picture below will be what needs to be changed to adjust the distance from the original impact location to the desired distance.
Make it a great day