Wheel Building - Double Snowflake

Now my problem is, i can not figure out the spoke length for my wheel build.
Standard 'flake is pretty easy, +2.5-3mm per full twist, but with this build the spokes travel aross the wheel left-right-left and right-left-right, so theres no "easy" way of guessing the length.

In theory, the right-left-right thing shouldn't matter and 3 cross plus a bit for the twists should work. The paths followed by the spokes are roughly the same: each spoke follows all the sections of a straight path, they just happen to get their full set from several different (but equivalent) straight paths.

You can see that the wheel in the OP hasn't been properly tightened as there should be the same angle between each pair of spokes on each side of each twist. If it was done up tight the innermost twist would move closer to the hub and it would be more obvious what is going on. Making the twists slightly closer to the hub than they should be might help them end up in the right place.

(all this is theoretical - my practical wheel building experience is just a few plain 3 cross wheels.)