I should add that my chart does obviously use the SAT2400 scale, meaning that the writing score is lumped in there on the SAT, while not the ACT. It still should be fairly accurate though, as I can't imagine a huge difference in writing scores versus the others.
The official chart uses the 1600 scale, and then a seperate writing.
However I should point out that for a truely accurate conversion, you really need to throw out the ACT science scores...
My percentiles are as accurate as you'll find though, they are rounded by at most 0.05%.