OK - then just make those adjustments when you make your custom entry for that bread. I eat a ton of fiber every day and I don't low carb. But, if I were to do that, I'd just add the bread and, where it tells you to put in the carbs, subtract the fiber grams from that number and put in your adjusted carb number when you define that food in the 'customized food' section of your log.
I think that would work. Then, in the future, when you logged a slice of that low-carb bread, it would automatically just add the custom entry carb grams that you specified.
The only problem is that you have to customize every single thing you eat that has fiber. For example - I'm not looking up the numbers, but... - say, you ate some garbanzo beans, which are high in fiber. You'd have to make your own custom entry for garbanzo beans and call it 'My garbanzo beans' and reflect the changes for net carbs in your entry. But in the future, when eating ordinary garbanzo beans, you'd have to use your custom entry for that.