So on average you backup 1271GB/Day. If you assume 500GB/tape (~1.2:1 compression) then you consume 2.5 tapes a day.
To keep a 60-day supply with your current settings it would seem you need 150 tapes minimum (2.5X60). Because you have four tape drives; they're likely all busy doing individual jobs each night. So, there is some job append inefficency and I'm sure there are some tapes that are not being used to full capacity. I would factor in 1 tape per night for this and add 60 tapes and assume you currently need 210 tapes to get your 60 day retention level.
Obviously you need to rotate tapes- but is your biggest concern the amount of tapes you're using or is it the time required to remove and insert tapes?
If it was me, I would want a simple rotation scheme that can be easily explained and maintained even if it cost a few more tapes than other methods.
I would partition the library into 2 sections: one section would be 20 slots and would be for Full backups only. 20 slots (10TB) should provide enough space for one each weekly full backup. I would create 8 sets of 20 tapes (160 total) and create a 2 month rotation scheme with these sets.
The remaining 54 slots would be for daily incremental (I don't think you can afford to do differential with what you have). These tapes would never leave the library but rather age out and get reused at a 60 day OPP. This scheme uses (160+54) at least 214 tapes.
You would need to change your media set settings to an OPP=60days, infinite append. You would need to create a library partition within veritas, and would need to define/re-define once/week full backup jobs to go to the 20-slot partition, and daily incremental jobs to go to the 54 slot partition.
Once each week, you would remove all of the tapes from the 20 slot partition and replace them with the next rotational set.
Again, this is based on some loose calculations and may not work as well when put into practice. Other methods would be to stop doing weekly full backups, and do only one or two Full backups per month and then the rest of the month do differentials or incrementals. (note: never mix differential and incrementals inbetween FUll backups when using the clear archive bit method). This would require less tapes, but it depends on how much your data changes each night.
If the time comes around when your budgeting for a new library, take the total amount of disk space you plan to backup and multiply by 10, and that would be a starting point for library capacity. For your situation, somewhere around a 200-250 slot library would have been nice. ;)