Sadly, even using that as an estimate it would have been off by more than half of how many would actually die.
It is also worth remembering that everybody thought the war in 1914 would be short, there were very few people who thought it would drag out for years. All involved probably did some mental math based on actions like the the Franco-Prussian war and figured that the same level of casualties, or even twice as many, was worth it.
If they would have known, which we do with hindsight, that it was going to take 4+ years of fighting it may have never happened.
I think anyone who thought it was going to be short, would have been completely ignoring all evidence that would have stated otherwise.
The biggest problem was that it had been over 40 years since Europe had been at war. This meant that there was little evidence based study to do with new military technology advances and it was too easy to dismiss colonial conflicts (Boer War, Russo-Japanese War) as not indicative of what a general war in Europe would look like.
It is actually remarkable to hear some people argue that a world war today would be short (if it is non-nuclear) because of the same reasons.
Yes. Germany came within spitting distance of Paris thanks to the Shlieffen plan, and was only hobbled at the eleventh hour by overextended supply lines.
Meanwhile, the Entente was counting on the Russian juggernaut, with its millions of men to overrun East Germany. The Russians saw great successes against Austrian forces, but met a complete disaster when they engaged the Germans. As it turned out, the Russian empire was a paper tiger - completely unprepared to fight a war.
Calling Russia a paper tiger is complely wrong, just because they were not quite as effective as Britain, Germany and even France does notean they were a paper tiger. Compared to Austria, Turkey amd even Italy they managed a efficent economy and had a huge well trained army. The problem was that the civilian government failed, the militay and economic side were suprisingly strong.
Unlike WW2, it was also not an existential war - it was just another European conflict. An armistice gets signed, the losers pay reparations, everybody goes home.
France may have been able to hold on for a while longer, but losing Paris would have almost certainly made an armistice the preferable option. It would not have been able to regain its territory, its industrial output and manpower would have been dwarfed by Germany, and its allies were in no position to offer assistance.
The war happened because of that, largely (and the related belief that it would not only be short, but heavily favor the aggressor—a belief often referred to as "offense dominance"—which is why the first beginnings of readiness in response to crises set off an unstoppable rush to war.)