Ebb & Gro bucket system - bucket spacing

I'm in the process of setting up the Ebb & Gro bucket system. My room is 9x9' and I've got 4x 600w lights. The strains are Sour D, Flo, and OG K(r)ush. (for more info see journal here: First Grow - 3000w - Ebb & Gro - H&G - Sealed )

I'm cutting the 3/4" tubing and trying to figure out how much space to leave in between the buckets??? The buckets are 2 gallons and the diameter of each bucket is 9.5 (25cm)".

I'm planning on vegging for 4 weeks and flowering for 8 weeks. I'm in week 3 of veg right now and the sizes range from 12" (30cm) - 18" (46cm). I figure they will probably be 18 - 24" (60cm) tall before I start flowering and maybe 48 (122cm) - 60" (153 cm) tall when I harvest. (it's my first grow so really don't know.)

Should I leave 6" (15cm), 12" (30cm), or 18" (45cm) between the buckets? 12" would give each plant about a 22" (56cm) diameter space to grow in.