If i understand well, you want to place windows of 100cm while there is available space in the spline, and fit a smaller one at end.
This can be done easily with the "Evenly" rule, because provides specific parameters to get this behaviour. I did it on the following way:
1) To divide the spline in sections of 100 cm. i assigned an empty segment to the Evenly slot. It doesn't add any geometry, but create the divisions. Distance = 100 cm., Adjust to End = 0%, Justify off.
2) Once divided, i created a Segment attaching the two sides of the window, and used it for the Default rule on "Scale" mode. In this way, each window is scaled according the available space. (100+100+... +space to end)
Benefits of this approach are that it works for splines or any length, and you can modify its behaviour with the Evenly parameters.
I hope that helps.