My approach is similar to others. Figure a good “in the ballpark” and add a couple links. Run through the gears. On the big-big, I’ll always stop the pedaling and see how much more travel I have on the derailleur stretch.
You have to remember that a chain climbs onto a larger cog/cheinring. In all honesty, I’ve never searched, nor tried to figure out, if there is a point prior to actually seating on a cog/chainring where more chain is needed than when seated. For me, I don’t want the derailleur so stretched out that I have almost no margin for error.
John