The off-the-shelf focus lenses are designed to take in a collimated laser beam (that means the laser beam is coming straight into the lens). However, in your setup you angle the incoming side laser beam because you use an upstream focus lens.
There are a lot of geometric optic software programs that will show what is happening, and a lot of youtube tutorials on the subject, and you could simply sketch it out by hand after watching some informative youtube vidoes then see what is going on, then change your design or lens configuration. You can also contact suppliers of lenses for their assistance.