Hello,
I have done a similar calculation. I found a formula for the breakeven point according to level. For level L, assuming that your damage is 1d8+L/3 or 1d10+L/3 (doesn't really matter, but I prefer twin bastards), the breakeven point is always 20. The L/3 factor is a guesstimate of the magic enhancement bonus of your swords, combined with the weapon focus bonus (which goes up by 1 every tier). You can replace the L/3 by L/4, doesn't make a difference. Your hunter's quarry can be 1d6 or 1d8, doesn't make a difference. I've taken into account criticals, as well as hunter's quarry. You have to be careful with quarry since it only procs once, even if you hit twice, so its TS calculation is less straightforward.
In any case, I found that if you need a 19 or 20 to hit, then CS deals about 91% of the damage of TS at level 1, and at level 30 this gets close to 95%. In other words, you lose 5-9% damage by using CS.
If you hit on a natural 20 precisely, then you gain a respectable 18% damage at level 1, and 29% damage at level 30.
If you hit on a 21 precisely (i.e., ts still hits on a crit only, but cs hits on 19,20), then you're losing again. This time, you're sacrificing 16% damage at level 1, and about 10% damage at level 30.
So if you know the exact AC of the baddie, and you know that you hit exactly on a 20, then by all means, use CS.
The rest of the time (if it's harder than a 20, or easier than a 20), forget it!
I have done a similar calculation. I found a formula for the breakeven point according to level. For level L, assuming that your damage is 1d8+L/3 or 1d10+L/3 (doesn't really matter, but I prefer twin bastards), the breakeven point is always 20. The L/3 factor is a guesstimate of the magic enhancement bonus of your swords, combined with the weapon focus bonus (which goes up by 1 every tier). You can replace the L/3 by L/4, doesn't make a difference. Your hunter's quarry can be 1d6 or 1d8, doesn't make a difference. I've taken into account criticals, as well as hunter's quarry. You have to be careful with quarry since it only procs once, even if you hit twice, so its TS calculation is less straightforward.
In any case, I found that if you need a 19 or 20 to hit, then CS deals about 91% of the damage of TS at level 1, and at level 30 this gets close to 95%. In other words, you lose 5-9% damage by using CS.
If you hit on a natural 20 precisely, then you gain a respectable 18% damage at level 1, and 29% damage at level 30.
If you hit on a 21 precisely (i.e., ts still hits on a crit only, but cs hits on 19,20), then you're losing again. This time, you're sacrificing 16% damage at level 1, and about 10% damage at level 30.
So if you know the exact AC of the baddie, and you know that you hit exactly on a 20, then by all means, use CS.
The rest of the time (if it's harder than a 20, or easier than a 20), forget it!