I need to pour a 4'x10' concrete slab at a 20 degree angle in front of my boathouse. How do I do this without having the concrete run over the bottom form? Any tips are appreciated.

Tell the driver you want a stiff mix and be ready to trowel a bit longer. 20 degrees isn't all that great and the shifting should be small, pour more on the high end and pull down to the lower end.

Not getting it hear maybe. But why not cap that edge(s)? Or is the "bottom form" the form on the lower side of the slope and that's that?

Can you be more specific in details?

AS bal1 stated Stiffer is good.