However dark as you like on the back three windows.
Don't bother with anything on the B pillar forwards (passenger and driver window, windscreen) as a certain % of light needs to be able to pass but your window will be factory tinted close to this spec anyway.
Afaik as long as the strip doesn't cover the part where the wipers sweep your windscreen then it's fine. I previously had one where it did and still never had problems with mot.
You wouldn't want one too big anyway as it wouldn't look right so that shouldn't be a problem