Okay, hopefully without talking too much...
If you have a belly pan under there and cover up the hole you won't have vortexes anyway. So, just throw some coroplast on there and don't worry about a radius. Flow attachment usually is a problem on the rear, but with a slope like that on the front everything that goes under the air dam will be completely detached. There's nothing for the flow to "ride" on.
With ducting you should be fine. The Probe is fine even without ducting. If you're not aware, P&G and EOC keep engine temps down pretty well also. Mutual benefit.
