MAS3:
Your voltage divider adds up to about a factor 11, not as a factor 10.
I guess that's what you are tripping over.
That's obviously not the issue if you look at the code.
float voltage_divider = (98700+9790)/9790; //((R1+R2)/R2)*voltage on A0-pin
One issue is that the calculation is in integer arithmetic, and secondly that error of about 2% is perfectly within spec anyway.
That code sets voltage_divider to 11.000.
float voltage_divider = (98700.0+9790.0)/9790.0; //((R1+R2)/R2)*voltage on A0-pin
Will do the right thing. Voltage regulators (such as the 5V regulator on the Arduino that you are using as
the voltage reference for ADC measurements) will normally have a +/- 2% or 5% rating - until you measure
the actual 5V rail voltage and substitute it for "5" in the calculations you can't do any better.