Depending on sea conditions, it should be enough to plane quickly especially when seas are relatively calm. If the wind kicks up a little more deadrise could be helpful.

The degree is based on the design method and the area of operation. there is no fixed formula for its calculation

Its variable from boat to boat and down to design and drive I'm assuming hope this helps

no it will give you a softride but very unstable in beam sea

Depends what you mean by better. The 17 degree deadrise will cut through the waves better than the 21. However the 21 will plane faster and will be more stable at rest. So better is dependant on the use you are planning.