How to determine sizes using sgdisk partitioning in bash script

I'm using sgdisk in a bash script similar to this: sgdisk --clear /dev/vda --set-alignment=1 --new 1:34:2047 --typecode 1:EF02 -c 1:"grub" -g /dev/vda sgdisk --new 2:2048:16779263 --typecode 2:8300 -g /dev/vda sgdisk --new 3:16779264:20971486 --typecode 3:8200 -g /dev/vda That works only when the devices are well known in advance and the sectors are hard-coded. I want to drop the hardcode the sector values. Instead, I want the script to work when the disk size is not known until the script runs. After making partition 1, I will set aside a known fixed amount to partition 3 for swap, and give the rest to partition 2. The easy way would be to make the swap partition #2. I know how to do that. However, I want to see if I can instead **do this while keeping swap on partition 3**. It means sgdisk will have to calculate a size or end sector value for partition 2 taking into account the size that will be allocated for partition 3 in the next step. Reading through the sgdisk man page hasn't given me the clues about how to do this (or even if it can be done).