The Compact Linear Collider (CLIC) study is a feasibility study for a new linear accelerator that aims to reach a center-of-mass collision energy of 3 TeV. To keep the length of the accelerator reasonable, a high accelerating gradient of 100 MeV/m is provided by a novel acceleration scheme, where power is extracted from a high-intensity drive beam to accelerate a high-energy main beam. The Test Beam Line (TBL) at the CLIC Test Facility 3 (CTF-3) is an experimental beamline constructed to test the technology for deceleration and power extraction of the drive beam. A Beam Loss Monitoring (BLM) system is currently under development to investigate the amount of beam loss at the TBL, with the aim of providing information about the stability of the beam under deceleration. These detectors are placed outside of the accelerator, and measure the secondary particle shower created by particles lost in the TBL. The amount of particles that can be detected by the BLM detectors was simulated using the Monte Carlo transport code FLUKA. Several different loss scenarios were simulated, in order to calculate the intensity and composition of the secondary particle shower at the detector locations. Various approximations for the sensitivity of the detectors were considered, and were combined with the simulated intensity of the shower to estimate the detector output signal per lost particle. These values were compared with data taken by the TBL BLM system, to estimate the amount of beam lost while the TBL is running.