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educate me here what does GF lo have to do with NDL dives? am I missisng something. As I under stand It gf lo is moot until NDL is over stayed and only the gf hi is of value of being used. That for a rec ndl dive there is no difference between 30/70 and 70/70. that gf hi of 70 adjusts the NDL so you can directly surface and hit the surface with a GF of 70 with no stops assuming you did not exceed 70 on the ascent. 30 being first stop and 70 being the max allowed GF through out the dive. Max or the highest GF is expected as you hit the surface.
Using deco planner.
100ft on EAN32, all plus or minus a minute, so don't be pedantic
With 30/70, you get 10 mins on the bottom
With 70/70, you get 20 mins on the bottom
With 99/99, you get 30 mins on the bottom
The gf-lo sets the threshold not to exceed for the first stop which is how it controls the stop depth. That will absolutely change your bottom time when that "first stop depth" becomes shallower than the surface.
Now what is worth noting and why I don't agree with what you guys are doing is this. If you do what you're doing when you say I want to surface at GF70, you are still exceeding GF70 during your ascend. If you're ok with ascending at GF99, then you may as well get the rest of the way to the surface since you offgas faster at the surface than you do at 15ft since the partial pressure of inert gas is still higher than 0.8.