How do I partition a drive to an exact size in OSX Terminal?

Question

I've got a 3TB drive partitioned like so:

TimeMachine 800,000,000,000 Bytes TELUS 2,199,975,890,944 Bytes

I bought an identical drive so that I could mirror the above in case of failure.

Using DiskUtility, partitioning makes the drives a different size than the above by several hundreds of thousands of bytes, so when I try to add them to the RAID set, it tells me the drive is too small.

I figured I could use terminal to specify the exact precise sizes I needed so that both partitions would be the right size and I could RAID hassle-free...

I used the following command:

sudo diskutil partitionDisk disk3 "jhfs+" TimeMachine 800000000000b "jhfs+" TELUS 2199975886848b

But the result is TimeMachine being 799,865,798,656 Bytes and TELUS being 2,200,110,092,288 Bytes. The names are identical to the originals and I'm also formatting them in Mac OS Extended (Journaled), like the originals. I can't understand why I'm not getting the same exact sizes when I'm being so specific with Terminal.

Edit for additional info: Playing around with the numbers, regardless of what I do I am always off by a minimum of 16,384 bytes. I can't seem to get the first partition, TimeMachine to land on 800000000000b on the nose.

Answer 1

So here's how I eventually got the exact sizes I needed:

1. Partitioned the Drive using Disk Utility, stating I wanted to split it 800 GB and 2.2 TB respectively. This yielded something like 800.2GB and 2.2TB (but the 2.2 TB was smaller than the 2,199,975,890,944 Bytes required, of course).
2. Using Disk Utility, I edited the size of the first partition to 800 GB (from 800.2GB), which brought it down to 800,000,000,000 bytes on the nose, as required.
3. I booted into GParted Live so that I could edit the second partition with more accuracy than Terminal and Disk Utility and move it around as necessary.
4. In GParted, I looked at the original drive for reference, noting how much space it had between partitions for the Apple_Boot partitions that Disk Utility adds when you add a partition to a RAID array (I think it was 128 MB in GParted).
5. I deleted the second partition and recreated it leaving 128 MB before and after the partition and used the original drive's second partition for size reference.
6. I rebooted into OS X.
7. Now I couldn't add the second partition to the RAID because I think it ended up being slightly larger than the 2,199,975,890,944 Bytes required (i.e., it didn't have enough space after it for that Apple_Boot partition), I got an error when attempting it in Disk Utility.
8. I reformatted the partition using Disk Utility just so that it could be a Mac OS Extended (journaled) rather than just HSF+, to be safe (matching the original).
9. I used Terminal's diskutil resizeVolume [drive's name] 2199975895040b command to get it to land on the required 2,199,975,890,944 Bytes (notice how I had to play around with the resize size, making it bigger than my target size to get it to land where I wanted).
10. Added both partitions to their respective RAID arrays using Disk Utility and rebuilt them successfully.

... Finally.