Guesses or information on the megaton yield of a photon torpedo

I am working on an adventure in the TOS era, and I came up against a problem. Can anyone guess or know what megaton range explosion results from a single photon torpedo? I found several references to photon torpedoes being 18.5 Isotons, or roughly 64.4 Megatons, and I think this is the TOS era. However, I have found several other references for Mark-6 torpedoes with much higher yields in the TNG and later era So now I am confused. One reference quoted that a Mark-6 photon torpedo could be dialed to higher effect, at the desire of the firing ship, UP TO 200 Isotons. Also, does anyone know when photons were first used in TOS? Guidance, oh wise ones! Thanks in advance.

The TNG tech manual says Photon Torpedos where first introduced in 2215. HOWEVER the current basic design of torpedo was developed in 2271 when they changed how the anti matter was packed in allowing for more efficant destruction and dialing of power. As for Power a Photon Torpedo can be as destructive as you want. I mean the Torpedo casing alone has a mass of 547 or so kilos. speed that sucker up to the speed of light and you’re hitting with a few hundred megatons I imagine. To be brutally honest warheads are a waste of time once a civilization reaches the point to accelerate an object up to the speed of light.

1 g Antimatter is roughly 43 kT. Roughly twice the yield of the “Fat Man” bomb dropped upon Nagasaki (21 kT)

TNG Tech manual lists 1.5 kg, so 64.5 MT

Keep in mind, the T tons used are equivalence to Trinitrotoluene (TNT)…