Yes, David, and you will too. Don't be scared of it, it's trial and error that will get you where you want to go consistency-wise.
One thing that comp cooks do is to try to find a fairly consistent brisket in terms of weight, proportion, and grading. This is usually achieved by buying by the case from a wholesaler, often one that supplies restaurants where consistency of product is much more crucial. The suppliers source with that in mind and many if not most of their individual products (above and beyond briskets) will be quite consistent in key areas. Trial and error as a learning tool becomes much more reliable (and easier!) when you are working with fairly similar briskets from cook to cook.
If that is out of the realm of possibility then try to find an actual butcher that can source for you. Tell him what you're looking for and why and see what he can do. However, a visit to area restaurant meat suppliers might be all you need to do (I find a visit better than a call). Some (many in some areas) will retail if you buy by the case.
It is not at all impossible to learn all you need by buying and cooking whatever is available at the local store. But you can focus more on honing an approach if your product is fairly consistent rather than honing an approach plus making adjustments because the grades are all over the place or size/thickness is erratic. That said, keep cooking regardless--you'll keep learning as you go no matter what you're cooking.