How can I set exact pixel or percent for color-stop position on gradients?

Question

I've created a rectangle and in its filling options I chose gradient:

Now my rectangle has gradient fill, but how can I set it to an exact value?

My rectangle is 100px wide, and I need 10% (10px) of gradient. I.e: from left to right until 90% it will stay solid black, and between 90-100% gradient effect will applied.

I can move the slider for color-stop, but can't set it exact (I'd like to enter the values). How can I do this?

This is my goal: having a solid black rectangle with exactly 10% gradient at the right side:

(This is my current one, where gradient is not exactly set to 10%)

Answer 1

2 rectangles butted against each other.

One 90px wide and solid back, one 10px wide and a gradient from 100% to 0%.