You need to place the PCB more or less in the middle of the oven between the furnace and the pre heater sensor. It is very important that the PCB is at least under the Furnace sensor so that this sensor is not receiving any radiation heat from the pre heater (bottom) IR-lamps. This to ensure that the temperature controller will function properly. (Very small boards can be placed on an older larger PCB or FR-4 piece of material.)