Long Enough for derailleurs?
Run the chain around the two biggest cog wheels, the fastest at the front and slowest at the back. Don’t thread it through the derailleurs. Find the shortest length that would connect and add on 1 inch, one complete inner and outer link. This is usually the ideal length for modern derailleur systems.
Short Enough for derailleurs?
Thread the chain through the derailleurs and round the two smallest cog wheels, the slowest at the front and the fastest at the back. Offer up the two ends of the chain and match the link that makes the chain short enough so that the bottom run of chain doesn’t rub on swing arm near the top jockey wheel on the rear derailleur.