There is something here @farmlife but I have a sneaky suspicion...

There is something here @farmlife but I have a sneaky suspicion this author has got confused with the ATR and true range - anyway might be a start.

Truerange:= Max( H - L, Max( H - Ref(C, -1), Ref(C,-1) - L) );

However, using algebra, it can be simplified to this:
Truerange:= max( H, Ref(C, -1)) - Min( Ref(C,-1), L);

Here's the publication anyway - must admit after all these years I've never read it or seen it before.

https://www.metastock.com/customer/resources/formulas/Formula_Primer_II.pdf