Good to see you again,
@Gudrun !
I think it will be difficult to calc the carbs in juice, because the pulp, and some juice will be left behind.
Personally, I would be tempted to just use the whole carb content of the fruit, and not subtract anything -erring on the side of caution!
Without the fruit pulp what carbs there are will arrive in the bloodstream faster than if you were eating the whole fruit/veg and chewing it as part of a meal.
They key to it working could be to test rigorously, before and then every 15 mins until you know how fast it hits the bloodstream, and how high you go. I definitely wouldn't rely on a 2 hr post prandial reading for juicing.
If you go too high, then you can make a few adjustments
Less fruit
Change to less sweet fruit
More veg
Stir some of the pulp back in to the juice
Add psyllium husks
Drink it slowly, over a couple of hours
My preference is for green smoothies (see link in my sig) because they are slower release, but I've done some reading on juicing, and there are some success stories out there, so you should be able to make it work with some tweaking.