Dell support do all the calculations for us. I have asked about how the allocations to RAID-10 and RAID-5 are done, but I haven't seen an answer. I think the calculations are proprietary and not divulged.
Basic working assumptions are that 20 - 30% of actual data on a VMFS datastore and hence a SC Volume will be at RAID-10, and therefore will occupy 2x the space on the SAN disks. This is hot data.
70 - 80% will therefore be at RAID-5, for RAID-5-9 the actual data will be roughly 1.13x when on disk. This is cold data.
Then add on top snapshot overheads. I reckon a single snapshot overhead should be about 20 - 30% of your actual data occupying 1.13x space on disk. This effectively a cold copy of your hot data, at a previous point in time.
These are very rough numbers, as I say we had to ask Dell for the recommended SAN size based on expected performance numbers we gave them.
We are not using a backup solution. None is required for the workloads we support. We do have very high redundancy on storage instead. Dual Dell Compellent SANs, each running in dual redundancy, with dual controllers, must-pathing and each SAN mirroring the other, with an independant tie-breaker node.