What is the best way to figure how much decay would happen for a particular day ?

For Intraday FnO position, what would be the best way to predict how much decay would happen in a particular strike price with the assumption that underlying stock remains at the same price for hours ? if this is possible at all( not sure as I do not have the best understanding of Option greeks).

This would help in knowing what stop loss would be good at Strike Price level for a corresponding movement in the underlying stock