Ha, you're right about that, I am definitely over thinking this, but that's what I do. I may be able to find a flat section, but it's more my whole house. If you just let go of a marble on hard floor, it starts acclerating pretty quickly. Remember, I'm talking about fine tuning here, so 3' isn't going to cut it anyway.
There's probably no solution for my particular situation. I'm just going to have to live with it or build something.
Let me ask you this though, if you wanted to practice just the line of your stroke (not so much speed) and you had the choice between a perfectly level 10 foot putt or one with an unknown slope, wouldn't you choose the level one? I would.