Pursuit gradient factors

VIKTOR66

GF is a % of the M-value. The low is the maximum % of the M-value of every compartment. If you remember the M-value is the maximum value for a given compartment (focusing in only one compartment for now). For example if you use a 15/85. That means during the ascent no compartment will be allowed to exceed 15% of its M-value. This will add more stops to your deco profile as you have less supersaturation available so the steps have to be shorter. Lower number GF implies more stops and starting deeper. Similar to VPM profiles.
The High GF is maximum supersaturation allowed at the surface. It means you cannot have any compartment when you arrive at the surface having more the 85% ( in 10/85 example) of their M-value. Lower number here means longer last deco stop as you have to wait there until all compartments are low enough. So there is not transition. High GF only applies in the last stop.

Now how to select the numbers.It will depend of the dive depths and bottom times you are planning.

If you plan a recreational dive no deco...then GF 100/100 will be ok theorically but in practice is not very conservative and some people more sensitive will suffer DCS hits. So using 90/95 (with air) will be reasonable.
If you plan a dive to 30 mts for 20 minutes, a no decompresion dive in any table if you use GF 10/50 for example your deco will be:
60 feet- 2 min
50 feet- 1 min
40 feet- 2 min
30 feet- 3min
20 feet- 2 min
15 feet- 23 min

Obviously an exorbitant and useless deco for a very well tested in the field no deco dive.
So to not bore you the GF have to be applied depending of depth and time and mix used. By experience from tech divers there are some values used in decompressions that should be personalized depending of factors as fitness, age, hidratation level...etc...

The typical GF could be

0 - 140 feet/ 40 mts you could use 90/95
140 feet/40 mts to 300 feet/90 mts I would use 15/85
More 300 feet/90 mts I would use 5/85 or 5/75 depending the dive

But this are numbers based on the experience of a few divers so they don't have a statistical validation yet but is coming as more tech divers are using them.

You can read Eric Baker's paper about deep stops to see some comparatives and explanations of GF.

I hope it helps