The easiest formula is how many records will each subscriber add into the database between syncs. Take that number and multiply by 5. That should give you plenty of identity values for a decent amount of time if the systems don’t replicate for a couple of cycles. Of course if you replicate every hour you will want to increase the number that you multiply by to say 24 or 48 to cover a couple of day window of outage.
You don’t want to go two crazy with the number or you will have so many values left in the range that when you replicate SQL will not realize that you need more and will not assign a new range to the subscriber and you’ll end up running out of IDs again.