I managed to reach the desired result, at least on the devices i had acess to, by changing line 170 "window.scrollTo(scrollStart.x *scaleChange,scrollStart.y*scaleChange);" to "window.scrollTo(window.innerWidth/3, 0);" at "in5.viewer.js"
But in all the files i came across the zoom goes top left and not center as mentioned.
Ps. Dualpixel made the question on my behalf