I probably doesn't do it correctly as it doesn't seem to work.

Here is my process:
I create a guide from surface. First issue, what if I want to create guide on surface manually? Do I have to pass through this modifier?
What I do is set the root count to 1 and add my guide with the plant tool from edit guide.

Sometime I have an error if I delete all guide and create with the plant tool.

Next if I change the num guide point in the guide from surface, it just affect the guide created from this modifier, not those created from the plant tool.
And an other thing, if I create a new guide with the plant tool and use interpolate strands, it will use the same Num segs than the interpolated strand. I'd like to have it interpolating the shape of the guide but not the Num seg.
And in the end I'm obliged to create new guide curve to get new num seg for the guide.
I was not able to chage the segment number from an existing guide.