It should always be given by weight. This is why making blanket statements about doses can be so harmful. You were smart not to give 500mg.I too was told twice per day, and hopefully my dosage I used on a silkie was correct as somewhere here I read 500 mg twice a day, so I gave her half that for a week. She just had come out of the coop with a horrible respiratory issue one morning that lingered for two days and I have no idea why she was the only one affected but this did the trick.
When my vets have prescribed it, it's been 125 mg/kg (57 mg per pound), and all veterinary textbooks I have seen say 125-150 mg/kg.