We have built the tool with some simple assumptions based on the FCA handbook, which states that the lower and higher estimates for the asset class returns must maintain a 3% differential to the medium estimate. (FCA Handbook COBS 13 Annex 2 Projections)
Once the 'Medium estimate' is added, the low and high estimates will get automatically added. An example is:
Asset class - if 6% is added for the 'Medium estimate' then the low will be automatically set to 3% and the high will be set to 9%.
We have followed this approach for the Inflation and yields section using the differentials that are shown in the above mentioned Annex.