There are two ways to use your configuration with manual control of power.
1. Set up everything for TTL then just switch the power settings on the DS 125 to 1/8, 1/4, or 1/2. Full will not work because in TTL mode the camera signals for a preflash and then a main flash and the strobe is not fast enough to fire two full dumps that fast.
2. Set up everything to TTL EXCEPT set the camera strobe setting to slave strobe. Then just switch the power settings on the DS 125 to 1/8, 1/4, 1/2, or Full. Full will work in this case because in slave mode there is no preflash.
That said, having shot manual for 100s of dives, I now shoot TTL exclusively. On any given shot, given enough tries, I might be able to beat TTL. But in reality, my shot opportunities are like pop-up targets. Point - shoot, point - shoot. No time to fool around.
For more on the settings I use, see the following URL.
http://www.splashdowndivers.com/photo_gall...aphy/index.html