The 'per' you refer to relates to measurement in relation to a dimension, time, volume, mass. Take four per day but take at most 1 every 6 hours. (you could use every instead of per) His fat index is 80 gms per kilogram. In formal dimension ratio phrases I think we'll always use per. 80 kph; 7moles per litre; 5000 milli amps per hour. that's probably a safe rule - if there are formal time/space/mass dimensions - 'per'

Per: it indicates "one" as a singular, for example, "you have to pay five dollars per day (each day)" Every: this one indicates everything as a plural, for example, "you have to pay school every day (all days)" Hope I clarified your doubt, if you need more ESL lessons I will be more than happy to help you, I am on Skype as UriQuintal, have a nice day! :)