I guess you know about the CtH formula;

CtH = 100 * AR / (AR + DR) * 2 * alvl / (alvl + dlvl)

AR = Attack Rating; alvl = Level of Attacker; dlvl = Level of Defender; DR = Defense Rating.

If you've studied algebra it's quite easy to solve the above formula for DR, multiply it with 0.75 (which is the -25%TD as long as there's no stone skin etc. involved) and put the number into the original formula. That still doesn't take alvl and dlvl into account though.

I suppose it's possible to derive a formula for CtH (including -25%TD) as a function of CtH displayed in the LCS (still disregarding alvl and dlvl). But the easiest way should be to just look up the monster stats somewhere, e.g. d2data.net, and just use the CtH formula (but of course multiplying the monsters DR by 0.75 first).