Standard Length is the more professional measurement. Tail fins are notoriously erodable.
Since you don't want to subject your fish to unnecessary net-handling, and you aren't measuring a thousand fish, lay a plastic ruler in the tank, get some milk and a plate of Oreos, and wait for him to measure himself!
That's why most in the hobby today only measure from tip of the snout to the area where the tail begins. In many books you will find the length listed as 5cmSL or 5cmTL. It does make a difference.
I use either the SL or TL method, depending on the fish. If it's got a relatively short tail or where there's not a really clear stop to the body and start on the tail I measure everything. Something like a fantail goldfish or betta, with REALLY long or feathery fins gets body-measurement only.
Usually I just eyeball the measurements (grew up in a household full of tailors, so I think I've got measuring tape lines permanently seared into my field of vision....) and round up to the 1/2".