Best practice of shrinking the transaction log is to shrink it all the way down to smallest size possible (close to 0) and then alter the log to grow it back to the desired size.

If the desired size is greater than 4gb, then alter the size in 4gb chunks. Per Kimberly Tripp (if I remember correctly), when you create the tran log greater than 4gb or alter it by more than 4gb, the vlf sizes are greater than what is optimal.