Hi Chris,
How about your license memory size? if you can use 128GB for HANA, then you can allocate limit 64*1024 for each instance, must not set 90% of physical memory.
It is setting base on your allow memory size of your lHANA licence.
If you can use 200GB, then you can allocate all memory except OS or other software used part.
Hope this can help you.
Regards,
Jerry