By default, each zoom level increases or decreases the zoom by 20%. As a result, you will get 100%, 120%, 144%, and so on when increasing the zoom level by 1. 

For positive values, setting zoom level to 2.0 means the page is scaled to 144%, for negative values -2.0 means 1.0 / 1.44 = 69.4%. To set a custom zoom level you should use Browser.ZoomLevel  property.

Converting Zoom Level to/from Zoom Percentage

To convert the zoom percentage to the Browser.ZoomLevel value you can use the following approach:

double ZoomPercentageToZoomLevel(double zoomPercentage)
    return Math.Log(zoomPercentage / 100) / Math.Log(1.2);

To convert the Browser.ZoomLevel value to the zoom percentage you can use the following code sample:

double ZoomLevelToZoomPercentage(double zoomLevel)
    return Math.Pow(1.2, zoomLevel) * 100;
1.2 is a Chromium default multiplier (equivalent of 20%)