Assuming 'RKL' - means 'Reef Keeper Lite'? And assuming you are trying to set your return pump to change flow often to create different flow? If you can set it that way, I personally like to alternate flow in different directions. I do not think its a good idea to vary the speed of your return pump - for a variety of reasons. Thus - I would tend to recommend if you want varying wave flow in your tank that you should get powerhead of your choice and set them up on an alternating pattern. Hope this helps!
PS - if you just meant you want to adjust the flow of your return pump rarely, Seems like you should be able to do it (but I would also wonder 'why')