Personally, I prefer to err towards the warm side. So, I would not be comfortable with 77-78 esp if my BP was hanging out on the cool side all the time.

If your snake hangs out on the cool side and its too cool you are risking him getting an RI - that's personal experience.

One thing I recommend is to cover three sides of the tank with rolled cork from the craft store. Its good looking, will make the snake feel more secure, and will boost your ambient temps by a few degrees. Particularly, it will help with cold spots in the corners.

Even with cork, I have found that I having some supplemental heating on the cool side is really necessary to push it up into the low 80's. I use a low-wattage heat lamp to accomplish this.

Also, I don't know how thick your substrate is, but keep in mind that temps under the substrate are likely to be a bit warmer than right at the surface. I anchor my accurite probe to the edge of the warm hide with just enough slack so the probe extends to the center of the hide. My snake actually lays on the probe when he's in there. In any case, you should take a few measurements around the cage. You might be surprised at how much the temps vary.

Finally, make sure the top of the tank is mostly covered. This helps keep heat in as well. Check out the glass tank sticky posts to see how its done.