800mg is prescription strength, granted I usually just take 2 500mg and a cup of coffee any time I have a headache. IDK if it is the caffeine or the ibuprofen, but any headache will usually be gone within 30 minutes.
Makes sense that my dr/nurse friends are the ones that told me to do 800mg when I use it (and it worked much better). I typically don't take painkillers, and ibuprofen isn't something to be taken everyday anyway.
My wife's doctor said the optimal does for an average adult is 600 mg (typically 3 pills) - he warned that going below that won't product the same results. When it's been prescribed, it's always 800 mg.