As a rule of thumb I go big to big not through the derailleur, see where I could join the chain and add a link, remember how many links I could remove. Then go small to small through the derailleur and see what the fewest links I can remove while there is still tension on the chain, usually the cage is horizontal. Somewhere between the two will work, I err on the shorter side but a lot of the time it's the same length with both methods. If you use all the fancy maths and whatnot it tends to be the same.
As a rule of thumb I go big to big not through the derailleur, see where I could join the chain and add a link, remember how many links I could remove. Then go small to small through the derailleur and see what the fewest links I can remove while there is still tension on the chain, usually the cage is horizontal. Somewhere between the two will work, I err on the shorter side but a lot of the time it's the same length with both methods. If you use all the fancy maths and whatnot it tends to be the same.