See further down in the post.
I went back and read how the OTU was calculated, and let's run some numbers. Per the NOAA Manual 5th edition, over 6 days of diving you are allowed up to 420 OTU doses average per a day and a total of 2520. At PPO2 of 1.4 you get 1.63 OTU/min. So even if you spend the entire time at 1.4, you need to dive 257 minutes a day, and a total of 1,546 minutes for the 6 day period. So over 21 dives that would be 73 minutes per a dive, which means unless you are diving EAN40 at 80 feet (using the NOAA Nitrox charts ver 7) for every dive you aren't likely to come close to it.
Whole body toxicity is only really a concern for rebreather divers because of the select able PPO2 set point. They can tell their rebreather to keep the PPO2 at 1.3 for the entire dive, including shallow portions. Which allows them to build up the toxicity. While recreational OC divers only really near their PPO2 max for a small portion of the dive. I am willing to bet if I went through my computer my time above a PPO2 of 1.3 per a dive could be counted on my fingers and toes.