Yes, it's slavish reliance on FMGC figures which are the problem! LIDO, like other suppliers, uses knowledge gained from years of experience to develop practical figures - such as the FL you suggest. But if the alternate doesn't have a STAR, or the crew doesn't have access to LIDO at the time? That's why they need an easy 'alternate fuel calculator' which doesn't assume that they'll be able to use optimum levels.