I think the 2.5 hours is a otu thing. with a conservative OTU limit of 300 minutes and diving to mod means you are at 1.4 PPO2 then 300 divided by 1.4 is about 210-220 ,,,,,, that is same as 300 minutes at 100% on the surface or 1.4 PPO2 at mod. 2.5 hours is 150 mi so it sounds like they don't want you to exceed more than 70% of your daily OTU limit. Is it important to comply with that value. I dont know. a 30% safety factor is a real protection margin in any corporate lawyers mind. Quite often recommendations and science do not go hand in hand, when legalities and liability prevention comes into play. I did some reading on this and 300 otu a day is a conservative limit if not the most conservative limit. I seam to remember something about 360 OTU's a day.... If that were used then they are trying to keep your exposure under 50% of the daily limit. 50% os not a risk at all. If that is true then this is run amuk over protection. I would rather them tell me it is a recommendaiton for the over all population not to exceed XX% of daily allowable limit of xxx. with out the assumed daily limit you dont know anything other than a value not to excede. its like saying drive no faster that 90% of the posted speed limit of 80. You dont know if that is mph or km/h or feet per second. Wording like that starts ringing bells like those that say must be O2 clean at 23.5 percent and nothing about the basis for that statement. A sheeple mentality.