With MGNLSTK-949 one can now configure the start level of the sub navigation which is a very cool thing, BUT: If you do not configure a start level and you do not have your home page on the top level, the start level of the sub navigation is wrong.

Steps to reproduce:
1. go to demoauthor.magnolia-cms.com
2. create a new top level node of the type redirect
3. underneath create a second redirect node
4. copy the whole demo-project site underneath the second redirect node
5. open the copy of demo-project and navigate to some sup page
=> The sub navigation starts with "home" (i.e. 2 levels off)!

The problem lies on line 87 of SiteNavigationModel where the startLevel of the sub navigation is calculated by simply adding 1 to getHorizontalLevel() instead of adding the startLevel of the horizontal navigation (as it was done in 2.0.9).