DaveSSS hit upon a key issue.

However you measure a chain there are three possible lengths. The method described will give you the shortest length, big/big +2 links. You can also use BCRider's method for the same results. You can use the method DaveSSS describes to find the longest length, small/small with RD just taking up slack.

If your RD cage is long enough, the shortest length will be shorter than the longest length, otherwise you have a choice, change gearing, replace RD with longer cage, or cheat a bit using the shortest length and allowing the chain to sag if you accidentally shift to the small/small combination.

In the event that there's a range between the shortest and longest length you have choices. Sram recommends the shortest length method, which will obviously yield a lighter chain. Campagnolo recommends measuring by the longest length method who's advantages are more chain wrap on small sprockets, freedom to increase sprocket size later, and for MTBers the ability to cut out a damaged link and resplice the chain.

When setting up race bikes sometimes a length between the shortest and longest length will allow the RD to be set higher improving shift response, and I try for the length which allows that using either chainring.

Regardless of how you measure it, consider big/big +2 links to be the absolutely shortest length. You may say to yourself I never crosschain, so I don't have to worry, but chains are incredibly strong and if you ever shift into a combination it can't wrap, kiss either your RD, wheel axle, or chainrings goodbye.