I work for a competitor to your stated provider, however, we give the option for the user to specify either 1500ft or Normal Cruising Altitude for Final Reserve Calculation.
In the second scenario, the system will review what your cruising level was for that leg and use that flight level to calculate the fuel burn.
I would assume it would be on this basis that your current provider will also calculate the fuel figure to be used.
I do agree that the regulation is not clear and we, therefore, implemented the calculation using this methodology.
If you have any questions, I would imagine speaking with your provider directly is the way forward.
I hope this helps.