Your approach will differ depending on whether you want your images to be cropped or scaled to fit the appropriate size, but generally speaking, in most of the image style effects where you can specify a width and height, if you just enter one value, the value for the other will be automatically calculated ("Resize" is an exception, but since that can result in distorted images, you almost always want to use "Scale" or "Scale and crop" instead.) For example, if you add a "Scale" effect, leave the width field blank and set the height to 150px, when processing a 400x600px image, the system will notice that it needs to divide the height by 4 in order to reduce its height to 150px, and apply the same division to the width, resulting in a 100x150px image.
Again, depending on how you want images to be scaled or cropped, applying more than one action in sequence may also get you where you want to be. For example, scale to a certain width, then crop to a certain height. The "Scale and crop" action can do both of these together in one action, but might not do it in exactly the manner you want.