Reverse Vertical Scrolling. What is the correct setting?

[raymundbiddi]

Guys, I need the reverse scroll feature for this plugin and have found in the CLOSED issues about setting a negative value for data-stellar-background-ratio to achieve this feature. However, I have noticed that my backgrounds are offset (thus showing a white background) and have to use data-stellar-vertical-offset="150" to show it correctly. My issue now is that when scrolling, the background scrolls too fast.
On a normal non reversed scrolling of the background, I am using a setting of data-stellar-background-ratio="0.9" which is fine, so I am assuming that if I ever want to make the background scroll in reverse, the value of -0.9 will do the trick. But this is not the case. Any of you know what additional setting needs to be added?
This is the kind of effect that I need to achieve: h++p://recettesapartager.fr/