I quit using HPFS, years ago. Two major reasons:
1: The HPFS cache uses lower shared memory space, which is often critical to get other things to work.
2: When a CHKDSK is necessary, at boot time, JFS is a lot faster, and less likely to corrupt something.
Since modern disks are not really limited by size, I use 5 GB for boot drives (4 GB is plenty, but what the heck), and I keep the UNIXROOT on the boot drive. I also do not split data and programs, to different drives, but that is up to you. You may want a FAT32 drive, to share data with windows (FAT32 is VERY slow, even when it is on a NVME drive). If you want to install an ancient program that balks at the free space, install it to the boot drive, then move it if you want to.