you're not doing deep deco, so I would not personally bother upgrading that spool, but I wouldn't deploy until I was at say 20ft. That's deeper than any boats that are going to be in that area, is a perfectly fine depth to make your stop, and doesn't require serious reeling during ascent. Anything that requires more than a 100ft spool with 75-80ft on it that is planned is done with a primary reel because it's easier to wind back for me.
only thing I would change from what tbone states is to deploy from 30-35 ft instead of 20ft....change in buoyancy will be a little more stable as you shift slightly up or down in the water column while sorting out and deploying the DSMB. There will be a greater swing as you shift from 20ft especially if you move upwards.
-Z