Exactly. Even within a single sheet, the dose per "tab" can vary. Outside of that, even from the same vendor, the dose might vary by a factor of 5. The only remotely reliable method is to take enough to be psychoactive, then wait a week and take it again, trying to hit the point where you have threshold effects (a binary search over dosing strength). Even then you might be off on your dose guessing be 30% or more.
He may have gotten far less LSD than he believes.