According to the AcrobaticsDDI skill you can use an Acrobatic Stunt to "somersault over an opponent", amongst other things . It gives a base DC of 15, and "the DM sets the DC based on the complexity of the stunt and the danger of the situation."
Rather than using a static DC for all opponents, I am thinking of using the enemy's reflex defense as the DC. Would this be a balanced DC that scales appropriately as the PCs and enemies level up? If not, what would be a better value to use?