Looked it up on the internet ...

(bulb watts / 4) x (ASVLS* - Lamp Clearance inches) = bulb temp at tip

*ASVLS = air speed velocity of laden swallow. Unfortunately there is a "y" factor in that we do not know for sure if we are timing an african or european swallow -- so results may vary some from reality and depending on headwinds

No, really, keep your lamp about a foot off the plants and keep the CFM's moving. I like to use a fan too. I like the leaves moving constantly as it prevents some burning that may occur from hot spots and such, and it strengthens the stalks to carry the heavy load