chain stretch's how much?

totally dumb question i know, but can't find a more specific answer.

so setting up my freshly hardtailed bike. got a new o-ring chain(drag spec) and wanting to know how far they will stretch, i can either sett up my wheel at the very start (looks kinda funny) of the dropouts or just past the middle of them. i read elsewhere that after 2.5% stretch it should be replaced. non-oring 102X2.5%=2 links (rounded) the only way to get a 2-link stretch would be setting the axle in the front.

every-one says they stretch but how much?

i would imaging im overthinking this and everyone sets them up center of dropouts. just dont wanna have to buy chains all the time. (less adjustment length)

thanks
ryan