Please don't misunderstand this post - I am a great believer in working things out for myself, and seeing what I think is the nub of the problem. The wizards are very good, and I use them myself, but - when things go wrong it leaves you high and dry trying to guess the problem.
Why are you bothering with the handy dandy trig table and the Wizard - you have the figures in front of you.
For every foot the taper is 0.59941 so the GCode is (from 0,0) G1 Z12 X-0.59941.
You cannot do a straight cut - you will have to do some roughing cuts - but it is as simple as that.
If you are doing anything short of 1 foot - say 3,4 or 6 inches, divide the amount of taper by 4,3 or 2.
I haven't checked whether the taper applies to diameter or radius, as Bill said - but you can just alter the lathe from radius mode to diameter mode - wthout altering the maths.
Now if you did that and it didn't fit we can find out why, but unless I take the wizard apart line by line, I don't know.
I'm sure Bill has the answer, and I am sure your second go will work, but try writing simple GCode, it is quite rewarding.