Others will probably chime in but playing with pretend data is one thing, actual data another.
Unfortunately this is a question only you can answer yourself.

You can only test dummy data so far.  Are you planning on others using it?  If so maybe give another person a test copy and ask them to help you beat it up.  No matter what I do it seems like I don't account for all the ways people can break things.
I think it is a case of me being to close to the trees to see I'm in the forest.

The next question is, are you going to have more than one person using this at the same time?  Once that happens it can cause other issues. Splitting your db.

The real world scenario is you will probably never be totally finished with your db (at least in my case).  Our db is 25 or so years old and I still tweak it on a regular basis. Making little changes here and there as I learn different things here.
The key is good back ups and working on a copy of the db.  I very rarely make a change in a live db.  That almost always is a disaster waiting to happen.

The biggest thing I've noticed is as Scott said, if others will be using it, you will probably find a bunch of errors that you didn't expect. It can work perfectly how you use it (the intended way), but somehow, someway, other users always find a way to get to stuff that you wouldn't even think it was possible (why would they do that?) and break something. I would probably go live alongside your excel files at the same time for a week or two and make sure everything is how you expect it to be.

You just have to keep testing and testing, and until the number of errors and problems go below what you deem to be an acceptable number, you keep on testing.