It is important to get your floor timber level to the horizon. On the picture you san see a method which I devised. You fix the floor to the chine logs with clamps. Then you clamp a sliding bevel to the tilt floor. Tighten the bevel so that you can move it by tapping. Then place the level onto the edge of the bevel and tap to perfection. You can use the fixed angle to draw the guidlines for planing down the timber. You can aslo check your planing progress with the angle.