How to calculate service ceiling and absolute ceiling in PA-44 Seminole?
You can use the given chart to define (single engine) ceilings. There should be separate graph for both engines running case.

Note: it is important to understand that the definition absolute ceiling is absolute: zero climb rate while service ceiling is defined by local regulations. 

In Europe, in commercial operations for class B aircraft (which Seminole is) service ceiling is the altitude where both engines running aircraft is able to climb at 300 ft/min at present gross weight and local OAT. A single engine service ceiling is defined as "positive gradient" so it is the first usable altitude below single engine absolute ceiling.

To define required altitude enter the graph from the right from the desired vertical speed, go upwards until you meet your current (or estimated) gross weight. From there draw a horizontal line all the way across the left side of the graph. The point where to local OAT (at an altitude) and your horizontal lines cross is the ceiling according to the selected climb requirement.

When interpolating between the altitude lines make sure to go perpendicular between altitude lines - not up and down along the grid.