I just finished filling my new 210. Attached the RODI output to one of the overflow bulkheads with a clamp, and let it run (waste water down the drain, unfortunately) for 40 hours or so, until it got to the 160 gallon point (about 22" in this tank).
I then added a full bucket of reef crystals, and a pump to mix. While that was going on I made another 50 gallons in two Rubbermaid trash cans.
Once the salt was mixed well, I added my sand and rock, and then pumped in the contents of the trash cans, plus another 10 gallons I hurriedly made up. Total system volume (including 35 gallons in a 55 gallon sump, and accounting for rock and sand) worked out to right on 220 gallons.