I can't answer your question but I'm told to follow this line of thinking... Calculate the exact day supply. So say 1 box of novolog pens will last for 50.5 days. I round down and use 50 days as the day supply. If the insurance rejects and states that they only cover 30 day supply, then I bill 1 box as a 30 day supply. They can't charge back because you can't dispense a quantity less than one whole box (though some places do but that's a different argument). If they only cover a 90 days supply then you need a new script.
For flonase and rescue inhalers I don't calculate anything and just put 30 days for everything. I've never seen someone run out before insurance would cover it using this rule but if the directions are less than a potential of 7 puffs a day you could technically run into issues because the actual day supply would be greater than 30 days. That being said, a lot of plans only cover 30 day supply anyways so I'm not sure it's even worth calculating.
I'm not a pharmacist but I can imagine verifying these scripts is annoying. Did the tech put 30 days because the insurance requires a 30 day supply or did they put 30 days because they didn't calculate it? You can't possibly be checking this for every script... I think the only thing you can do is verify it and make sure your techs go about it correctly?