Usually optimal distance is specified by the manufacturer of the range hood ... 24-30 inchew is typical http://www.bestrangehoods.com/resources/range-hood-installation-tips.aspx

Hello, Thank you for your interest in our work. The space is 36" .

The hanging height of the hood must be determined by considering the the BTU's emitted by the cook top and the hood specifications. There is no "usual". That is how you get in trouble. I suggest you hire a good kitchen designer in your area!

Thank you. Going with a Blue Star 36” 6 burner range top with a 42” under cabinet hood and the placement is 30-36” recommended.

We are in final design phase of our kitchen and hope it comes out half as good as this one design.