I asked this same question a few months ago and a coach (rednu) posted this response and I thought it was very useful...
"You might be reaching too high for your D2 guys -- do a D3 search for 4-year players and take note of the highest overall rating that appears on that list. Add 25 points to that number and that's the upper end rating you can probably realistically expect to get a backup message from on a local D2 prospect, so if the highest rating nationally on your D3 screen is 450, I'd fire off a note to anyone inside my 300-mile circle whose rating is 475 or less (actually, I'd extend a point or two beyond this, but I've never had a kid beyond the (D3max + 25) range give me the time of day...I'm still hoping for the day I get confirmation that I can expand that range further)."
One thing to add; I don't fire off a note to every D2 prospect in my 360; just the ones I like..