Sorry for bumping.
Ok, I did a comparison between the OP's Excel VT calculator and Fridger's Excel VT calculator, and noticed that the +/- designations in the "error on X" in the OP's calculator appears to be inverted, where a negative number should really be a positive number, and vice versa.
For example, I have a tile size of 4096 pixels, and Fridger's calculator tells me to put my texture at X: 2099, Y: 2538, whereas the OP's calculator gives me a X error of -51 and a Y error of -490, which means the OP's calculator is telling me to move my texture to the left instead of the right. The Y error of -490, of course, tells me to move my texture down(/south), and is thus accurate.