I charge by the pane. It makes my life easy (for me) and also tells me how long it’ll take. Double hung $6 x 2. Casement $6. Big picture window maybe $12.
For businesses with residential windows on them… it depends on the level of service they want. You’re probably not washing screens and you’re trying to get them on a 2 week or 4 week schedule. So you could split the difference between storefront and residential.