The amount you add depends on how far to the side of the window you want the curtains to hang.
Using one curtain panel per window.
Purchased curtain panels always look skimpy to me because i like to use fabric generously and have nice full pleats.
Yes i think a room s look can be ruined by the use of skimpy curtains.
They can be hung with the tie on the right or the left of the window.
For covering windows however single panel drapes have a variety of uses.
Yes one drapery panel per window would look out of place.
One curtain panel on a window would look funny to me unless you could tie it back on one side but it would need to balance out with the other windows.
For example the left curtain panel should be drawn to the left and the right curtain panel should be drawn to the right.
For smaller windows where a double curtain will just be too much a single curtain panel is a much more elegant choice.
The rule of double the width for the fabric needed is important as is the use of the two curtains.
Usually curtains are hung in pairs but an asymmetrical look can be very attractive and so you may want to hang only a single curtain panel.
Long panels should look as if they are full enough to pull across the entire window.
Another option for a room with two windows is to pull both panels to one side and hold in place with a tie back.
If you haven t chosen one measure the width of the window and add 1 to 3 inches for each side of the window.
If you are splitting up a pair and only using one on a window i think it will look even skimpier.
Several single panels can be combined to curtain an extra wide window.
So no i don t recommend it.
Can be a roman shade style or hanging straight down from the bar and pair them up with two windows to draw the eye to the center of the wall.
Usually the drapery panels should equal two and a half times the window width.
The second window should be a mirror image of the first.
A single panel may also be more cost effective for.