No, there is no routine provided for this. You need to calculate the height at which the fill percentage is reached, then create a new polygon and fill that.
Calculating the height at which the fill percentage is reached is not trivial for irregularly shaped polygons, especially if the polygons are not convex. For example,
| |
| |
| _____ |
| | |_____|
|_______|
the area on the right cannot fill until the water has raised to a certain level, but once it has raised to that level the water does not start rising again until the right fills up.
You can easily see that you could create two chambers of the same volume with a small gap connecting them near the top; if the fill is into one side only then "almost half full" for the reservoir as a whole would involve having one side completely filled while the other was empty.
If you were to restrict to convex polygons the calculations are easier but still need to proceed piecewise to each change in angle of the sides.
