No formula, just heat it into the mid 170's and sparge. There is nothing particularly magical about sparge temps unless you are doing long fly sparges where continued conversion might make the beer drier than desired.
Something is always fermenting....
"It's Bahl Hornin'"
Kegged: Sour Saison, Pale Ale, Aggie Ale, Firestone DBA, De Koninck Blonde
Bottled: Belgian Quad (Grand Reserve), Derangement (Belgian Dark Strong)
On Deck: Pliny the Younger