Well, depending on your HU, you won't have an all powerful xover, but it will do the job quite well. I'm guessing you HPF can be set too 50, 80, 125 (standard for HUs, I think or roughly at least). Usually a setting around 80 or so will do the job, considering most subs will play well up until about 90-130Hz. But then again, I shouldn't be answering this question cause setting a cross over is all about the equipment you have and how you like your music. I set mine by ear, not by any kind of books. It will depend on what kind of front speakers you have, how much mid bass they can put out,.. where they roll off/get distorted.. etc. Same for the tweeters and the sub too!
It's not an easy task, but using the HU you have limited options, so I would say just try different combinations of things. But once you have/if you have a full system that is completly powered by external ams and such, I would seriously look into an external crossover if you want to really tune it. And beyond that are sound processors/eq's, and many other things. It's not an easy task to set it all up, but it can be fun too!