I'm not sure why you would not follow the advice from the manufacturer of your rear derailleur. Shimano puts it in their DM's and other documents. https://si.shimano.com/#/

Even for the same DR, Shimano sometimes has different procedures to determine chain length depending on how many chainrings the front has.

However you are essentially wanting the chain to be long enough to go into the big/big combo without pulling the rear DR cage to it's forward limits or to the point of it tangling up with the cassette. AND, you don't want it so long that when in the small/small combo the rear DR cage can't take up the chain slack and is on it's rearward limits of travel.

Though it seems like with some old Shimano DR's with a 3x front they actually told you not to use the lowest gearing. This might be when the mantra and dogma of "Don't cross chain or bad stuff will happen" came into wide spread misunderstanding.

Normally I've found that running the chain over the big/big combo without going through the rear DR and adding a couple links works great for the bikes I've had. And seems to be the most universal way to size a chain.