Pegasus RAID Useable space

I'm doing some drive budgeting for an upcoming show, and was wondering if there was a standard amount of headroom on RAID storage.

I'm looking at 2 possibilities for main storage drives. Both the Pegasus Thunderbolt2. In either the 18TB (6bay) or 24GB (8bay) configurations, how much USEABLE space do you get out of those after formatting?

Thanks.

Don't forget that a 4TB Raid in Raid 5 yields 3tb of space - essentially you lose a drive to the Raid 5 configuration. Not sure on the bigger sizes, but at least one drive is redundant. As to headroom - I'd not want to go above 80% capacity to ensure good speeds. I've had them over 90% but you really start to feel the slowdown. I doubt you'd saturate the TB2 bus with either.

To confirm Jason's numbers, I have a Pegasus R6 with 6x3TB drive mechanisms. The formatted capacity of my RAID5 volume is 15TB as reported by OS X Finder.

Although I have not let my RAID get anywhere close to capacity (yet) I'll echo that the general consensus is to not go too far over 90% capacity.

Thanks. I think I'll have to move up to the 24TB RAID. My calculations put the total assets (RED material + Graphics) at somewhere just over 14TB. Too close for comfort.

* Edited to add- Pegasus' site offers some guidance here.

Their TB1 6x3TB RAID is listed as 14.4TB formatted.
TB1 6x4TB RAID is 19.2TB formatted.

Would the 8x3TB version for TB2 offer any measurable difference in total formatted capacity than the 6x4TB?

8 x 3 = 24, minus one drive, you have 21 TB usable storage, rounded, same as a 6x4.
The only advantage of more drives for the same capacity is that more drives offer you slightly faster performance.
This held true even on the enterprise level XSAN 32 drive RAIDs I used to integrate.