Measured in a continuous climb. Normally you can extrapolate data to higher altitude linearly. This would give service ceiling of about 13,5 km (44 300 feet)! But sadly this is not true now. The rate of climb should decrease linearly when altitude increases, but because propeller is optimized for low altitude tows the real service ceiling is not that high. Airspeed (IAS) for best rate of climb remains about the same as altitude increases (decreases a little) but the true airspeed (TAS) increases. And propeller senses TAS, so we end up with a situation where engine overrevs. And we can not use full throttle setting. So we have to make a real high climb to find service ceiling.
Estimating that the overrev dilemma will limit service ceiling to something like 6 km. But with a higher pitch propeller we could reach stratosphere.