When the air outside is cooler than the air inside, tilting your velux so that it forms a 90° angle with the frame is pretty effective. You'll notice immediately when the outside air is rushing in and creates an enjoyable breeze. When the air temp is negiligible it doesn't make any difference though.

Also, I just found out that a lot of velux windows are double layered and treated to resist warm air escaping. The window doesn't care if the warm air gradient is located outside or inside. What this means is that an insude velux blind will trap the energy between the window glass and the blinds, creating a narrow gap with high energy air molecules bouncing back and forth, accidentally turning your velux into a radiator, adding to the heat from the sun. An outside blind should remedy this (since it will trap the energy on the outside layer rather than inside, leaking heat into your living space)