Formula rents 1999 valuations

Has anyone else noticed that the indices used to provide 1999 valuations seem to have been adjusted and now give much lower relative values than previously.

Up to 2015 the HPI was published and for example if £100,00 in Dec 2015 then divide by 193.4 and multiply by 92.5 to give 1999 valuation of £47,828. (ie 47%). But the values I now get from our Property team are about 29% of the current valuation, which even accounting for uplift since 2015 still gives a much lower 1999 valuation and therefore rent.

This only came to light recently as we have been building for affordable rent.