New Bolting: Stud & Nuts (rather urgent)

also i forgot to ask what is the extra length of 3mm for? is this the extra length of free threads?
because I have never done flange bolting before and I'm not sure what the standard is. In the link below supposedly its 1/3 the bolt diameter
http://www.wermac.org/bolts/bolts_method-for-calculating-stud-bolt-lengths-for-flanged-connections.html

also i forgot to ask what is the extra length of 3mm for? is this the extra length of free threads?
because I have never done flange bolting before and I'm not sure what the standard is. In the link below supposedly its 1/3 the bolt diameter
http://www.wermac.org/bolts/bolts_method-for-calculating-stud-bolt-lengths-for-flanged-connections.html