How to zoom images to exactly 50 %, 200 % etc.

In the default image viewer of Trisquel 7, you can set the zoom level to simple fractions / multiples of 100 % by holding Ctrl while repeatedly pressing the plus or minus key on your keyboard until you reach the desired level.

The above does not work with Viewnior, the default image viewer of Trisquel 9. Instead:
For 200 %, press 2.
For 300 %, press 3.
I think this is an undocumented feature; I didn't find any help for Viewnior other than "man viewnior".

For 100 %, press 1.
To fit to edges, press F or 0.

For others, like exactly 400 % or 50 %, I have no idea.

I hope this is useful to some of you; if there is a more appropriate place for posts like this, please let me know so I won't waste people's time.