GetTotalSpace returns the number of 512 byte blocks. So, you take GetTotalSpace/2 to get Kibibytes (KiB), or GetTotalSpace/2/1024/1024 for Gibibytes (GiB). GetTotalSpace for my 2 TB drive returns 3903979520, or 1861 GiB.
I have had a chance to look over your drive layout. It looks like you added two new mfs pairs, the first one ~1024GiB and the second one ~800GiB. This has put the base address of the second added pair starting at block 2159525952. Your version of mfs only stores a 32bit value for this, and it is above 2147483647, which is the maximum for a signed 32bit int. I would suspect that any tool that doesn't read this as an unsigned int would have problems. I read that you have filled the drive, so you appear to be safe there. I would be concerned about a mfscheck, have you triggered a GSOD and see how it is handled? It looks like you have a spare 2TB drive, maybe that would be a good test for you to run on it to make sure you don't run into trouble.
Another thing to test, is try adding the smaller pair first. This would keep the next pair under the 32bit signed int, and might be handled better by some of the mfs tools. That seems safer to me, but I am by no means an mfs expert. Maybe someone else will chime in on it.