I have a 20 amp circuit using 12 gauge wire. There is a total of 1000 watts of lighting on the circuit now and the "run" is approximately 65'. I want to put a single gang outlet and add about ten 25w to 40w flood lights to this circuit. My plan is to drop some conduit in a trench about 30' long and put the receptacle and the lights at the end of the new wire. My question is how much can I put on this circuit without overloading the circuit and what is the formula for figuring this out for future reference?

This should be what you need. To know if you are safe at 80% of the loading on your 20 amp circuit you need to assume that at some time all lights would be on at once. How To Convert watts to amps .: Cannaversity