There's no need to write path (I guess that's "formula" you mean) in the object editor,you must write path when importing icons and it will auto appear in object editor.
For BTN icon,set: ReplaceableTextures\CommandButtons\BTN[icon name].blp
For DISBTN: ReplaceableTextures\CommandButtonsDisabled\DISBTN[icon name].blp
Those paths are always written in icon description on hive.