The grade is meant to take away excess water, a 15 minute downpour might night be enough to notice a great deal.
I can guarantee you if you have at least 1/4" fall in every foot away from the house then your on the right track, it's even code compliant
. Anything more than that is a bonus, but from your previous posts getting more would be very difficult.
You can put down plastic to prevent 100% any water getting onto this area, but you will then have problems with "rushing" water and washing out any mulch etc...