yes, so here are factors that will cause me to drop my GF high
Didn't sleep well
Drank too much *of the decompression aids the nights before...*
Didn't drink enough *water*
Cold-both air and water. Cold water slows down N2 intake, but also slows release and since cold is cumulative, it slows progressively throughout the dive. If diving in water below about 80F I will usually dive a GF of 75-80, below 60, it is no more than 70
Work-if I am drift diving or on a DPV, my body is in a very different state than if I am kicking in current
How do I feel that day-I could just be feeling a little weird, tired, sore, etc etc and that will cause me to pad decompression.
Being deco certified, I can't tell you the last time I paid attention to NDL's. If you are doing a 3-5 minute "safety stop" you're already performing decompression, you may just not have theoretically saturated your tissues yet. Removing the stress of worrying about your NDL's allows you to lower some of the time pressure associated with diving deep. You have three factors that will cut your bottom time. Time, Air, Temp. If you remove time, you have cut your stress factors considerably and you just plan your ascent based off of a predetermined pressure. Much less stressful than having to watch both, especially if you violate that NDL as many will start reeling in their own heads about having violated that limit and think they're going to get bent. You get to the surface with a sore knee and think you were bent despite forgetting you knocked it into the bulkhead that morning. Have a rash and think it's skin bends even though you might have had a slight reaction to the neoprene, etc etc. It's funny, but by controlling the conservatism on the computers and KNOWING what it does and why you chose to set it at that limit, you are making the decision.
Quick example that are numbers taken out of thin air, so don't run this into pastodeco or something because I doubt they'll work.
Which is safer?
NDL limits at 100ft for 30 minute bottom time, GFhi set to 95, and you perform a 5 minute "safety stop" *keep in mind the Shearwaters do not force you into safety stops so they are not factored into your NDL's which may also be contributing to the extra conservatism. If the recreational computers have a 3 or 5 minute stop factored into their NDL's, that will extend bottom time*. You surface with a theoretical GF Hi of 80
Same dive, but your computer is set for GFhi of 80, and you blow your safety stop, make a slow ascent to the surface and come up with a GF Hi of 80.
Which is safer? Which is less stressful? Which has higher risk of DCS? You have to analyze yourself and determine which you deem safer. I will always choose the second dive profile, but others may not. I always have the option to increase the conservatism on that profile by adding a safety stop that will lower the theoretical tissue loading before surfacing. I also have a bit of a buffer at the bottom if I have to stay down for an extra minute or two due to an emergency without running into mandatory decompression.