My experience with that self leveling stuff is that it is very thick like pancake batter and doesn't automatically level like water. It sets up within minutes. You usually have to spread it with a float(such as a magnesium float) and that is dependent upon your experience, ability to eyeball it, and expertise with hand troweling. Like mudding drywall, skim coating plaster, etc., it is not easy for the the first time DIYer.
A large area is harder to hand trowel. You could get a straight 2by for a screed to level it but bathrooms are tight areas and you don't have forms(like with concrete) to hold your screed board so you are relying upon your skill to free hand it. Some level string lines positioned across the area probably can't hurt except for getting in the way.
If you don't like the job, chip it out and start over. I've also found that tapered thin edges (unlike drywall compound) will tend to crack on you. Good luck.