I am using 3950 thermistors for my hotend and found tables from RepRap. I have included the tables for the original Robo thermostat and the RepRap 3950. I checked my hotend with a Fluke 189 and a type K thermocouple. Worst case error from 170C to 240C was 1.4C. This is better than the manufacturing tolerances and I was pleasantly surprised. If you are not comfortable inserting a table into the Marlin code then just remove an existing table an put this one in it's place. At the top of the table I have given the number 14. Change that to the table number you are replacing and rename to the replaced table name.