Residual impacts from rainfall deficit and failures of traditional crop insurance to protect smallholders are resulting in economic losses threatening Post-Paris adaptation target in Nigeria. Weather index insurance is being considered as a possible pathway towards sharing farmers’ risks. This paper analyzed the reliability of rainfall indexes as proxy for calibrating grain crop losses in designing index-based insurance product for farmers in Central-West Nigeria. Results reveal that two strongest correlated rainfall indexes in the development phase of maize are the second dekad cumulative rainfall climatology and cumulative rainfall below the pre-set 440mm threshold, however it was the third dekad cumulative rainfall climatology and consec...