I feel the need to point out that the TFTD method is most definitely
not recommended, for 3 reasons:
1) It disrupts the balance - you can't harm tough enemy units with anything short of best weapons, because you can't count on a high roll near 200% (the "headshot principle").
2) While it is counter-intuitive, it is kinda unrealistic. Units in X-Com are vertical cylinders, therefore easier to hit than a human, because they're uniformly thick and also a bullet cannot go under their arm or between the legs. Very low rolls kinda offset this effect, thought admittedly by countering abstract with abstract.
3) In my personal opinion, the TFTD model makes combat kinda dull and unexciting. It just loses a bit of the "war" feel it's supposed to have.
I am presenting these three reasons to justify why I haven't used the 50-150% model myself. The current system is not ideal, I'm aware of its shortcomings, but I haven't found a better alternative yet. (Still considering various options.)
Of course, feel free to experiment, whatever works for you!