Be aware that mainly agencies or counties just take the state protocols, which are written for a large group of providers. I'm sure the states perspective is that most providers have relatively short transport times, like under 30 minutes. So if the nitro works, three doses should keep them pain free to get to the hospital, and if it doesn't work, they should be trying something else.
Also not to beat a dead horse, but nitro doesn't decrease mortality. So why the big push to be able to give lots of nitro without calling in? Are you really so busy during your hour transport time that it's a huge pain to call?