Generally for fiber, you just subtract total fiber from total carbohydrate, and that gives you net carbs. This is only true if you're in a country where fiber is included in the "total carbohydrate" count. Occassionally people will mistake this, and think that if they take 5 grams of pure sugar, and add 5 grams of pure fiber to that, they'll get zero net carbs, but that's not how it works, that would be 10 grams for total carbs, 5 grams net.

Technically, soluble fiber can result in some proprionate being produced by bacteria in the colon, the body can synthesize glucose from this, this wouldn't happen with insoluble fiber. So the "true" digestible carb count might go up a little bit with soluble fiber, but the absolute contribution from this is probably fairly small.