Are you talking about paper piecing? Then the enlargment percentage should work.
However, if you are talking about piecing a block, it's different. Usually you can find free block patterns online in the size you want. That will give you the dimensions of the cut pieces.
When you want to enlarge a pattern like this yourself, it's probably best to work it out on graph paper. Going from an 8" block to a 12" block, you would first draw the block on graph paper. Calculate how large each finished piece is, then add 1/4" to each side of each piece. For example, if a finished square in the pattern is 3", you would need to cut a 3.5" square (.25 added to each side of the square). Try this first with a simple block, such as a 9-patch (maybe going from a 6" block to a 12" block) to get the hang of working with the graph paper.
Or, invest in a quilt design program such as EQ7 or Quilt Pro. With those programs, you can automatically re-size a block and it will tell you what size to cut all of the pieces.