I fly a lot. I abide by these two rules: The space under your seat belongs to the passenger behind you. The space in front of you is fully yours, up to the farthest point back that the passenger in front of you can recline their seat.
You can use it while they don't recline, but it's theirs if they choose to use it. It has sometimes limited the angle on my laptop but that's what you get if you don't opt for first class.
I was on a SW flight recently and placed my laptop under the seat in front of me. That person decided to recline, and I could not bend over far enough to reach my laptop. It was kind of funny. I'm rather tall though so my situation may not be the norm.