So after looking at how the training is set up I noticed that the Ninja training schools initial trainings for levels 250-300 drop down to 1 red codestone with a train time of 4 hours. Which compared to the main schools trainings for levels 80+ is quicker. So I thought maybe instead of doing 1 level, 2 strength, 2 defense, and 2 endurance that possibly leveling to 250 and then doing all of your strength, defense, and endurance trainings using the Ninja school could be quicker and cheaper. Attached is the spreadsheet I put together. I couldnt figure out how to embed it here in the thread and keep the format. So if a mod can feel free to add it.
So based off how this is set up, assuming you have an equal chance to have your training require any of the codestones I used the average codestone cost retrieved from the Neocodex Item Database. This assumes that each "level" costs 7 trains, 1 for the level, 2 for strength, 2 for defense, and 2 for endurance. We are assuming no training bonuses. Using the traditional method it turns out to cost you 46.8 million NP and 872 days of training time. Using my new proposed method of focusing entirely on level until you reach Ninja School then leveling all of the others (strength, defense, and endurance) up to that point it will cost you 29.1 million NP with 369.75 days of training time. So based off this it would seem that instead of the traditional leveling method most people use may be both more expensive and more time consuming to this proposed method. I welcome you to double check my math and plan. I threw this together fairly quickly so I may be missing something here.
tl:dr Instead of leveling level, 2 strength, 2 defense, 2 endurance, rinse repeat method leveling to 250 to unlock Ninja school and then leveling up strength, defense, and endurance to 500 each may in fact be both quicker and cheaper.