The size of the polar cap is very important for understanding the substorm process as well as reconnection rates in general. In this work we build on previous studies which use a combination of European Incoherent Scatter radar (EISCAT) electron temperature (Te) measurements from two radars running simultaneously to track the motion of the open-closed field line boundary (OCB). The second radar gives an estimate of the background variation of Te with altitude, which can then be subtracted from the radar beam being used to estimate the OCB location. We demonstrate that using the international reference ionosphere 2007 (IRI-2007) model can remove the second radar requirement and therefore increase the number of cases which could benefit from background Te subtraction. In this paper we focus our analysis on substorm intervals. We find that the IRI-2007 method produces an OCB proxy location which on average is 0.25° altitude adjusted corrected geomagnetic coordinate latitude equatorward of the two-radar method. On comparing both the two-radar and IRI-2007 Te OCB finding methods with the OCB identified in the DMSP particle data and IMAGE satellite data we find that both EISCAT methods perform quite well, and neither method is particularly favored over the other. We find that the magnitude of the mean offset to the IMAGE OCB varies between 0.1° and 2.7° latitude, dependent on the event and the IMAGE camera used.