With WCDMA networks being deployed in Europe and throughout the world, one of the big challenges is to make cell reselection and handover between GSM and WCDMA work smoothly. Inter-system cell reselection between WCDMA and GSM enables the user equipment in idle mode to choose a new cell of another system to camp on, thus providing service availability when WCDMA coverage fades out. In this paper, we study inter-system cell reselection parameter settings by processing field measurement data collected in different networks. Performance metrics (relating to service availability and WCDMA idle mode coverage) are computed with our simulation platform to evaluate different sets of parameters in WCDMA-network-boundary, coverage-hole, and entering-a-building scenarios. Based on the simulation results, we discuss optimization trade-offs and recommend a set of parameters for each scenario