fractional reserve is not necessary, it only applies to volumes with LUNs in them. The purpose of fractional reserve is to guarantee that no matter what happens with snapshots, they wont take a volume offline. If volume runs out of space it goes offline to protect itself.
Either use fractional reserve 100% or use less, you can even go with 0% as long as you configure snapshot autodelete and set triggers so it happens correctly. This will ensure if snapshots are eating up space before volume goes offline ontap will delete snapshots. There are policies on what snapshots it deletes first but it will delete enough snapshots to keep volume free space enough to keep volume online.
Make decision on what is most important saving space i.e. cost or deleting snapshots this will guide you to decision on fractional reserve 100% or 0%.
Keith