Need proper series 380 (or 385) for several windows

Do you have existing balances? The best way to go about this is to identify your old balances and use the same ones when you replace them.

Are you confident that the plastic attachments should have wings on top and bottom like the Series 380 and Series 385? If so, you need only measure the bottom shoe as shown below, which will tell you the Series. The Series 385 has a 1-3/32" shoe, and the Series 380 has a 1" shoe.

Then, you can find the stamp on the old balance, which tells you which length and option to purchase. Is there a stamp?

You can simply weigh the sash. Using the area gets funky, between the glass thickness, number of panes, material of the frame, etc. Weight is a 1:1 match.

Each item webpage (S385-20 or S385-15 or whatever you end up needing) has a chart in the product description that tells you which option to choose for your particular sash weight.

Find a set of balances that seem to work with the sash correctly. The, get the glass height of the sash, and use the two lengths to pick a length for other similar windows in the house.

Example: if you have a 27" balance channel with a sash glass height of 26", you add 1" to the glass height to get the balance channel length. Don't forget to weigh the sash for the spring weight, as well.