I run about 78% nitrogen in all of my tires....

Here's my take on nitrogen in tires.  Air is 78% nitrogen, to fill with 99% nitrogen takes carefull purging and filling.  And that only takes care of 50% of the tire... the inside..  The other 50% of the tire is exposed to  weather, sunlight, and 21% percent O2 plus ozone and other stuff.  I haven't seen any cracks or checking on the inside of an aired tire yet that held pressure.  Even the tire that had the sidewall cracked, with the tread flopping off.

One is better served at least cost, with monthly inflation checks.  Use the money spent on Nitrogen tire fill on a small compressor & gauge instead.