Please accept my apology. It was unfathomable to me that someone who owned both a subal and nexus housing would not know what a guide number was. I went back and looked at all your posts this year and you clearly are a sensible fellow who does your part to help other folks on the site. I was clearly in error and since everyone I've ever met from Australia was a "good bloke" I will assume that you got up on the wrong side of the kangaroo this morning and I'll try again. From your info on the site it looks like you carry a Ikelite D125. The manual for that little beauty can be found
here. If you look at the first page you'll see these specs:
Weight: ......................................................1.2kg (2.7 lbs.) with NiMH
Energy Rating: ..........................................110 watt-sec.
Coverage Angle: ........................................90°
Widest Lens: ..............................................Nikonos 20mm or SLR 24mm
Nikonos 15mm or SLR 20mm with diffuser
Guide Number (ISO 100) feet: ................64 surface, 32 underwater
Guide Number (ISO 100) meters: ............20 surface, 10 underwater
Firing Modes: ............................................TTL/Auto, Manual, 1/2 M, 1/4 M, 1/8 M
Slave featuring auto exposure using optional DS Sensor #4100.5
Slave featuring 10-manual power settings using optional EV-Controller #4100.6
Color Temperature: ..................................4900°K
Power Source: ............................................7.2-volt, 2.8 amp NiMH pack
Flashes: ......................................................250 full power
Recycle Time: ............................................1.0 seconds (full power)
Depth Rating: ............................................90m (300 feet)
We'll come back to the Guide number. If you look on page 20, the guide table I was refering to in the previous post is there.
It says; quoting from the manual,
"The exposure label information is primarily intended for use with traditional film
cameras since digital still cameras may require less light for proper exposure. The
exposure label is compatible with TTL/Auto and Manual modes and references the
DS125 strobe output without the diffuser installed.
So right away they are telling us it really is a guide and not a formula or rule. It will vary with conditions. Some digital cameras will be different. Then they give examples;
TTL/Auto Mode example for 100 ISO film speed:
If you select f-8, you can shoot subjects up to 1.2m (4’) away.
If you select f-4, you can shoot subjects up to 2.5m (8') away; however, underwater
we recommend selecting subjects no farther than 1.8m (6') away.
Full Power Manual Mode example for 100 ISO film speed:
If you select f-8, the subject should be 1.2m (4’) away for proper exposure as
indicated by the letter M.
There are 3 positions on the 125 Full, half and qtr. Each one is 1 f/stop so they give this example;
1/4 Power Manual Mode example for 100 ISO film speed:
The difference between each power setting is one full f-stop; therefore, moving
from Full to 1/4 power decreases the light output by two full f��"stops. If you select
f-8, then reference two less f-stops (two numbers greater) than f-8 in the chart,
which is f-16. Subject should be 0.6m (2’) away for proper exposure.
So here is the rule; Guide number / distance = f/stop
Your D125 has a guide number of 10 underwater using meters (ISO 100) (This info is from the front page of your manual). Lets see if we get the same numbers as the table. So 10 div by 2 meters = an aperture of 5
Sounds about right. A couple other things to keep in mind. The guide number refers to the strobe to subject distance not the camera distance, so if your hand holding or doing macro it's the strobe distance that counts. One other important point then I'll shut up. The actual flash on time varies with the power. You'll notice on your chart that there are minimum distances. The bar doesn't go all the way from zero to 2 meters. If you are too close with too big an aperature the strobe cannot turn off in time(Quench) so you will be over exposed.
I hope I got this right. If not somebody will correct me.
Then there is of course you can use 'half power manual' but if you put the diffuser on, this will drop it by one FStop...well what the hell was it to start with... 
