HAT at minima, convert @317ft/nm,convert to metres, add 160m ... that's only approximate.
Wouldn't you be better using: HAT (on QNH) corrected for sea-level surface temperature, divide by 317, add 160m plus 18 * surface gradient of the HIAL; divide that total by the number of HIRLs you can see, add your birthday and then add the last 3 digits together ... that will give the day of the week for Christmas Day next year.
Of course, one of those little solar powered calculator makes this formula quicker - you'll have to be prompt, nimble of fingers, and well practised, you don't have much time from 200' AGL at 150k - but that the difference between IFR and VFR pilots.
On second thoughts, maybe a battery powered calculator would be better - gets dark at night.