The ceiling is the altitude at which it is impossible for steady level climb. This means there is no excess power at this altitude. The available power is tangent to the required power curve at minimum power.
The ceiling is obtained by calculating the maximum rate of climb at various altitudes.
This data is extrapolated to zero maximum rate of climb. This is the absolute ceiling.
For performance comparison, the data is extrapolated for maximum rate of climb of 100 ft/min. This is called the service ceiling.
In preliminary design calculations, the curve of the maximum rate of climb with altitude is almost linear. Therefore two data points are usually sufficient for this extrapolation. Since performance at SL and cruise are significant, these two altitudes usually furnish the data necessary for extrapolation.