[TheForge] slitting and drifting help needed

[robert hensarling]

Guys, is there a reference, or chart maybe, of what size slitting chisel to use for a given drifted hole diameter?  In other words, if I want a drifted finished hole size to be 1/2", what size of slitting chisel would I use, not only for this size, but 3/16th's, 1/4, 3/8th's etc. I'm referring to the "canoe" shaped slitting chisel.
 I understand that a slit and drifted hole will increase the overall length of a given piece of steel, and a punched and drifted hole will in fact be a little less.  Do I have this correct, or am I backwards?  
Thanks for the info!
Robert Hensarling
Uvalde, Texas