Yes, improper lawn slope can cause basement flooding issues. In fact, poor yard drainage is a common culprit to basement leakage issues. Instead of rain flowing down and away from your foundation, your yard acts as a funnel and brings water into your home.
This is especially frustrating when heavy rains occur. The rain falls onto your yard and rolls down towards your foundation.
The first step is to verify that this is the culprit to your basement leakage issues. There could be other issues involved, such as gutters and downspouts clogging or not pushing water far enough away.
In order to fix the lawn slope issues, you will need to make sure that the highest point of the yard is closest to your home, and that it has a funnel downwards and away from your home. This requires heavy equipment and machinery. This is not a one man with a shovel kind of job.
We always recommend consulting with a professional before making any changes to your yard, as the work done may not actual help solve the root issues.
As mentioned before, your basement flooding issues may not be the grading at all. It could be due to the fact that there are gaps in your soil that are allowing water to seep through and run up alongside your foundation walls or floor. The solution to this is backfilling dirt into these open areas in your soil. Again, we recommend hiring a professional to figure out which issue it is you are dealing with.
As a percentage, you generally want your yard slope to be at least 3%. It can definitely be higher than that, but anything lower than that may be risky.
A good idea the next time it rains heavily is to take a peek outside your home's foundation. If there is standing water, it may mean that your yard needs to be re-graded. It could also shed light on some of the other issues mentioned earlier, but either way, standing water next to your foundation is not good news.
The best time in the midwest to regrade yards is summer through fall. The ground is the softest, and it will also allow you to lay grass seed or sod before it gets cold again.
This is a tough question to answer without seeing each case individually. Oftentimes fixing yard slope issues involves a bit of both - adding to the top and removing from the bottom, and then leveling it out. This is another time we would recommend consulting a professional before digging in. It may save yourself a lot of headaches.