what's so hard about entering one number? I don't think anyone can say for sure whether the IRS would raise a question when you file 2020 entering the 2018 carryover when you entered nothing in the 2019 return. what happens if you can't use it in that year? current year FTC's are used before carryovers. if the IRS does raise an issue then you may be required to file an amended return.
to be brutally honest over the years I've have worked on hundreds of returns with FTC carryovers. I can remember only a few where they ever got to use any of the carryovers. In my experience, 99% expire without ever being used.
