John:
Although the basics are similar, the specifics of SCI and Douglas are significantly different for Asian water buffalo.

SCI score is all measurements added together, Douglas score is the sum of the smaller measurements doubled.

Douglas thereby penalizes an uneven or asymmetric trophy.

Also note that with buff horns, the Douglas length measurement goes from a point squared off with the open end of the horn, to a point around the tip and squared off with the centre of the rounded tip.

The point squared-off with the open end of the horn may be an inch or two out in space beyond the actual beginning of the leading edge of the horn. Easiest way to start the length measurement is to place the horn on the floor and push the end against a wall, with the horn at right-angles to the wall before curving away. The end of the tape is then held against the wall.

This sounds complicated, apologies, but it is quite simple in practice.
If I get time in the next day or so I'll scan a page out of the Douglas measuring manual and toss it up.