In this paper, we present the results of Monte Carlo simulations of γ-ray and cosmic-ray proton induced extensive air showers as detected by the TACTIC atmospheric Cherenkov imaging telescope for optimizing its trigger field of view and to pological trigger generation scheme. The simulation study has been carried out at several zenith angles. The topological trigger generation uses a coincidence of two or three nearest neighbor pixels for producing an even ttrigger. The results of this study suggest that a trigger field of 11x11 pixels ( ~ 3:43° x 3:43°) is quite optimum for achieving maximum effective collection area for γ-rays from a point source. With regard to optimization of topological trigger generation, it is found that both two and three nearest neighbor pixels yield nearly similar results up to a zenith angle of 25° with a threshold energy of ~ 1:5 TeV for γ-rays. Beyond zenith angle of 25°, the results suggest that a two-pixel nearest neighbor trigger should be preferred. Comparison of the simulated integral rates has also been made with corresponding measured values for validating the predictions of the Monte Carlo simulations, especially the effective collection area, so that energy spectra of sources (or flux upper limits in case of no detection) can be determined reliably. Reasonably good matching of the measured trigger rates (on the basis of ~ 207 h of data collected with the telescope in NN-2 and NN-3 trigger configurations) with that obtained from simulations reassures that the procedure followed by us in estimating the threshold energy and detection rates is quite reliable.