As you probably already know, the key flaw to percentage-based FILEGROWTH is that over time the increment grows larger and larger, causing the actual growth itself to take longer and longer. This is especially an issue with LOG files because they have to be zero-initialized before they can be used, causing excessive I/O and file contention while the growth is in progress. Paul Randal (blog/@PaulRandal) describes why this is the case in this blog post. (If you ever get a chance to see it Paul also does a fun demo in some of his classes and talks on why zero initialization is importan, using a hex editor to read the underlying contents of disk even after the old data is “deleted”)
Andy also has a script to change filegrowth to fixed-increment growth depending upon the size of the file, so check that out.