Mostly from observation of systems in the field, though it is a very hard to gauge number. I had one install where the I/O rate of the disk subsystem was dramat

Mostly from observation of systems in the field, though it is a very hard to gauge number. I had one install where the I/O rate of the disk subsystem was dramatically faster than the query plan was estimating (it was going too far to avoid I/O over forward read speed) and I discovered that there was absolutely no way to control its setting globally, instead having to add forced-index hints.