MT=a + b * log2(D/W + 1)
The basic idea that comes from Fitts’ Law is the farther away a target is and the smaller its size then the more difficult it is for the user to correctly land on that target. Of course, while this has to be balanced against other UX considerations, it should be part of the UX toolbelt."Thumb Zones"
The use of thumbs is a common interaction on mobile devices. There are times when the thumb is not used, but when we’re using our thumb we have to consider that the original Fitts’ Law formula applies only within the range of motion of our thumb. The problem occurs when an item falls outside of that zone.Portrait
Landscape
Source: http://webdesign.tutsplus.com/articles/design-theory/applying-fitts-law-to-mobile-interface-design/