There was no need to yell at the dealer, of course a lot of them need it. That is normal practice for most dealer mechanics. They have a tendency to put the initial adjustment of the chain tight. It will stretch a little.
Make sure you get some good chain lube and oil it faithfully every 500 miles(or whatever interval your manual says)
There is a lot of threads here on cleaning but thats another subject.
If your chain is at 3/4 you are probalby OK. Just go ride and oil it at the 500 mark. Oil it sooner if you get caught in rain.
Remember this is just one TBA riders opinion. Also IMO, Use PJ-1 Blue Label for O ring chains.