Hi,
I don't have a suitable environment to validate it, but looking at the scripting code for action com.vmware.library.vc.storage.computeDiskCapacity (which is used in presentation logic of "Add datastore on iSCSI/FC/local SCSI" workflow) it recommends disk capacity using the formula (return value is size in gigabytes)
return capacity.block * capacity.blockSize / 1073741824;
It seems to compute the size of the whole disk, but I suppose there is some amount of 'reserved' space that is not taken into account.
So I'd recommend to use a slightly modified action that assumes some blocks are reserved and returns a slightly lower size, something like
var reservedBlocks = 2; // not sure how much is reserved
return (capacity.block - reservedBlocks) * capacity.blockSize / 1073741824;
You can experiment a bit with the value for reserved blocks. I'm not sure how much space is reserved in case of VMFS 5; you can try to search if there is some documentation or google on Internet.