I have to admit
I just took a random length out of the chain until it seemed right??
Well, that's all you can do, but even if you use a half link the smallest adjustment you can make will be 1/2" in chain length.
I didn't put this very clearly before, I know, but this 1/2" in chain length has the same effect as moving the wheel forwards or backwards 1/4" (approximately). Now, as with a typical chainstay length/ring size combination something like 1/8th inch of wheel fore and aft adjustment alters vertical chain slack by nearly 3 inches, you can see that a half link will only get you "in the ball park" but not guarantee a perfectly adjusted chain.
With respect to anyone who runs lots of chain slack (and gets away with it !
) I absolutely would not do this or recommend it either. With your bike on a work stand I'm sure that you could run it like this for ever, but in the real world where a slack chain is going to be slopping about sideways as well as vertically it's a risk not worth taking, for the sake of your testicals, your face, whatever.....
Sod the aesthetics (a slack chain isn't aesthetically pleasing either, mind you) and run a decent push up tensioner or use an Eno rear hub or an eccentric BB adaptor. You only need to buy them once and it would be money well spent for the extra peace of mind.