Modify/customize existing cover entity

Hope this is a short one for anybody smarter than me.

A have this cover entity cover.shades_room and when the shades are fully closed state attrubute is 0 and when fully opened state attribute is 100.

What I try to achieve is customize that entity to show me the state attribute reduced by 1 and be able to use the new cover.shade_room_modified as my go-to-entity.

Why? See my other thread maybe: If I close the shades to 0 with the actual cover entity they tilt to horizontal and closing them makes no sense. But putting them to 1 will keep the tilting status tha is usually fully vertical.

This is giving me the correct state attribute but I can’t figure out where to put it. customize.yaml? But how? Full new template cover? Hope not.

{{state_attr('cover.sonnenschutz_schlafzimmer', 'current_position')|int -1}}