Hi Andi,
Great to see that you are trying to use the VNAV function.
As a quick rule of thumb, most airliners will descend at about 3 degrees at idle thrust and cruise speed. .
If you assume that, then the rate of descent needed will be approx 5 times your ground speed.
The distance needed will he approx 3 times the height difference (in thousands of feet).
So, to lose 10,000ft you will need about 30nm, as you already calculated.
Assuming your ground speed to be about 430kts, you would need a VS of 430x5 = 2150fpm to descend at 3degrees.
Hope this helps?