A lowpass filter is supposed to let in low frequencies and elimiate high frequencies. So the wikipedia version looks right to me- for a low frequency, dT should be big, for a high frequency it should be small.
However, I'd really suggest either hiring someone who knows DSP or doing some more research before blindly implementing a filter. These things are quite complex and a simple filter will often have negative side effects. You spend a year or so studying these if you become an electrical engineer. Odds of getting it right the first time on your own are fairly low, unless you use a library. And then the risk changes to using the wrong type of filter or configuring the constants wrong.
The filters shown by Wikipedia are about the simplest possible and have the most side effects, FYI.