Sizing vertical blind width so that windows are not obscured when blinds are pulled back.

[Ken Durchgang]

Hello - I want to put a split vertical blind over a window that that is 220cm wide, the blind will be outside of the recess. What width of vertical blind do I need to ensure that when the blind is pulled fully back on both sides that it doesn't obscure the window? I appreciate that this may vary a little by manufacturer. I've contacted a couple of suppliers but was surprised to find that they are unable to tell me this info about their own products. Is there a rule of thumb that will give a good estimate of the width required. I don't want the windows obscured when the blinds are pulled back but equally I don't want to go wider than necessary. Thanks.