Calculation of RNRB for downsizing addition

HMRCs RNRB calculator is producing an unusual result I don’t trust. Does anyone know the actual means of manually calculating the RNRB downsizing addition?

Facts are as follows:

  • D sold houseboat in 2016 for £47599
  • D died in 2025
  • D did not own a property at date of death
  • 50% of the estate passes to lineal descendants

The calculator is telling me I can claim £73,189 RNRB, but I cannot reverse engineer it in any way to arrive at that result. Can anyone help with this?

I also thought the allowance was capped at the value of the property? Does this not apply for the downsizing addition?