The only things to come from the tech manual were the total yeild and the size of the warhead. The rest was simple math: take the total result in megatons of yeild for the size warhead stated (assume a 100% efficient reaction) and divide by the stated number of isotons.
Indeed. But the yield in isotons is not supported elsewhere, least of all in aired Trek. And while we can "objectively" decide that an annihilation mass of 1.5 kg (or 1.5 + 1.5 kg, the wording is a bit unclear) is perfectly reasonable even if it's not canonical, we can't similarly deduce our way through the mystery that is isoton.
But the idea that isoton~megaton is workable as regards canon, so the TNG TM figures need not be completely out of whack. It just appears curious that the torpedoes of Voyager would have such massively larger maximum yield in "Scorpion, pt II": 200 isotons!
Unless, of course, the yield in that episode was for the sum total of the 32 warheads mentioned, giving just 6 1/4 isotons per torpedo. Which might be perfectly reasonable, too: the Intrepid class might carry featherweight ("class 6"/"type 6") torpedoes as standard, as opposed to the much heavier ones of the Galaxy class.
Indeed, if the standard Voyager torps were of 6 isoton (that is, 10 megaton) range, it would be all the more natural for Kim to gape at the supertorp in "Omega Directive", with some 50, and later 80, isotons. Not quite a planetcracker like he quips, but still very impressive for Kim's standards - as opposed to a situation where the standard torp would already be 25 isotons.
Timo Saloniemi