Add a YearMonth dimension to your data model.
This is to ensure that you only have one value per month. If you can verify that you only have one [Inception Date] per month then you can skip the part where I create a YearMonth field and use Inception Date instead.
Data:
LOAD [Inception Date],[Rate Variation],Premium, Monthname([Inception Date]) as YearMonth
FROM SourceTable;
Use the following expression in Qlik Sense KPI object.
=avg({<[YearMonth] = {'$(=maxstring([YearMonth]))'}>}aggr(Rangeavg( Above( total avg( {<[Inception Date] ,YearMonth >} [Rate Variation]), 0, 3) ), [YearMonth]))
Use the following expression in a chart where you have YearMonth as a dimension.
=avg(aggr(Rangeavg( Above( total avg( {<[Inception Date] ,YearMonth >} [Rate Variation]), 0, 3) ), [YearMonth]))
