Over the range means over 1100-(-270)=1370 degrees.
The error of ±1.5% applies to the whole range, so that’s ±1.5% per 1370 degrees, or ±1.5/1370=±0.001095% per degree.
At 950 degrees, the error is ±950*0.001095=±1.04% or ±0.0104.
In absolute terms is 950±9.88 degrees.