@Dayna19 I would also find a dog food bowl encrusted with dog food to be a cause for complaint. I'm a dog owner and even I find that disgusting. My dog's bowl gets washed daily.
And you should be able to remove the hard water stain in the sink with muriatic acid or CLR.
I can also see why a guest wouldn't feel secure if the back door and windows don't lock or were left unlocked.
I don't see any of this as unreasonable complaints, even if previous guests weren't fussed about it.
I agree with @Lisa723 that 50% would be a reasonable refund.