Hi Martin, still waiting for longer coax for 6 el. version 
in think that antenna will be complete this week. Inbetween i made 3 el. yagi DK7ZB (used your formula for scaling down) with 50 cm boom with elements divided to half. Transport length is 30cm. Here are pictures.
Balun:
Collapsed antenna:
SWR:
I made also some minor changes in joint construction. The holes for wire are in 45 deg angle now. There is lower lost of the length of wire for element .
73 Ondra OK1CDJ