Right, and here's a progress picture.
I designed my own sleeve decrease (based on another similar top pattern from Elegant Knit), which in Japanese reverse engineering would look like this:
The Japanese decreases (or increases) are coded with an 'one off' number of stitches to start with (optional), followed by several 'three numbers' sequences.
The 'three numbers' sequence has the following meaning : first number shows how many rows to knit after the last decrease, second number represents the number of stitches to be decreased and the third one tells you how many times to execute this decrease instruction.
So 2-3-1 means : knit another 2 rows - decrease 3 stitches - do this operation 1 time(s).
I've done a detailed chart as well, just so we see what's happening in real life, row by row.