My idea for a single finger zoom in/out is like this.
Firstly some "terminology".
Half tap -> touch the screen keeping the finger on the screen: used for panning
Tap -> touch the screen then immediately raise the finger: used as normal click
Double Tap -> touch the screen, raise finger, touch the screen again, raise finger: used as double click
My idea is ot use the "missing" Tap and a half:
Tap & 1/2 -> touch the screen, raise the finger, touch the screen again DON'T raise it
Once a Tap&1/2 is being detected the tablet switches in "zoom mode" and a white spot appears under the finger and a ruler could show up crossing the screen horizontally. The white spot is the "zero" of the zoom scale (i.e. 1:1)
As you drag the finger along the ruler, the more you go farther rigth from the white spot, the more the zoom in. Viceversa along the left side of the ruler for zoom out.
This can work everywhere and it's nice and intuitive to me.