First I don't think it is fair to the store to return a used bulb just because you want a larger one. Keep it as a spare. You should always have at least one spare bulb so you never have an issue like you just did. I always had more than one to be safe.

Are we dealing with a Ball Python? You said "basking spot." A BP needs a warm hide more than a basking spot. This can be accomplished by simply placing a hide under the lamp. You still want to know the actual surface temps you are creating and a IR temp gun is the best tool for that.

As for the uth, is it on a thermostat? If not, UNPLUG IT NOW. It can serious harm your snake without being regulated. Also the substrate over a uth should be very thin. I only use a 1/4 inch and the surface temp should be measured on the actual cage floor directly over the uth so you know what the highest temp the snake can reach is. They can and have burrowed down to them.

As to needing a larger bulb I cant really guess for you. In my old place my BP tank was in a room that stayed 68 or lower. I had a 100watt che, 60watt bulb, and a uth to keep the temps right in my 40 gal tank. Depending on your room temps you may need less or the same kind of setup. That is something you have to figure out.

As for the temps the air temp can be 75-82F and a hotspot in the area of 88-92 in recommended. Some keepers use a 95F hotspot but I see no reason for it as a 90 works perfect. Like I already mentioned getting an IR temp gun will make figuring all of this out much easier for you. They can be had cheap on Amazon.