What is the easiest way to be able to round, as normal, except to round down
if the answer is .5, 1.5, 2.5 etc?

So for example, if I want to take 90% and 10% of 5, using the normal ROUND
function I would get 5 and 1 for a total of 6 which is greater than the
whole. To fix this I'd like to have one of the formulas round down if the
answer ends in .5 (but not .6, for example). Thoughts?

Thanks for the help