No, you can't. Because reasonable is a subjective moving target. There are maybe a handful of organizations outside of Microsoft that have a non-laughable chance of estimating A) cost of online storage to microsoft B) number of users C) distribution of storage used by their userbase D) cost of bandwidth E) typical bandwidth profile of users F) how much money Microsoft is willing to spend on storage as a loss leader.
And then there is the un-estimateable wildcard: G) what number will "sound too big" to sysadmins and/or PMs once they see it on their metrics dashboard.
"Unlimited" is a scam to avoid vocalizing the secret maximum and hoping that users have more conservative estimates of the secret maximum than it's actual value.
And there is a huge incentive for users to guess conservativly at the maximum, because as soon as one person doesn't everyone else will blame them for "shitting in the pool and getting the whole thing canned"