Hi,
I think what you were describing is that the upper edge of the cheek piece is not straight and has a slight upward curve at the end? If that is the case, just use the rounded side of a half round file and gently straighten the edge. Draw a guideline on it first so you can visualize "straight". The flat face of the cheek piece often will widen toward the rear as yours already does. It does not need to be the same width for its whole length, although that is OK as well. Here are examples of cheek piece designs. Ignore the carving and inlays and just focus on how the wood is shaped.
As you can see, there are many ways to shape the cheek piece and no hard rules except to make sure it works to make the stock fit you. It can have an even height, or rise a little toward the rear. The edge can be an even flat or widen toward the rear, or have decorative lines and grooves, or just be rounded over. You get to choose.
dave
