I've made a calculator to compute ranking points on DMD. http://www.jerhu.net/games/dmc4/
Just select the mission and input values for orbs, time and style points. You can of course select the bonuses and penalties.
It will show you how much points you will get for that mission, using the famous equation : (Clear Bonus + Stylish Points) x Clear Time Multiplier x Orbs Found Multiplier x Bonus/Penalty Multipliers x Difficulty Multiplier = Devil Hunter Points