That is entirely fair. My numbers are based off of an ideal situation. No dying in battle. No major illness or injury to shorten lifespan. No curses. None of that. You die of ripe old age in your bed.
Also, I think that was supposed to be 240+80, which is 320. It's still somewhat off from the implied numbers in ShB, and what was previously said in a lore panel about Elezen lifespans though, and I suspect a bit of a lore inconsistency there. For my money, I'll roll with the Elezen live an ideal 120 years, multiply by 3, and round up to 400 total as they live 'easily' to 3 times the Elezen lifespan. Which would make 200 middle-aged and a reasonable point for Viera to start showing their age a little.