The Coppock is a long term momentum oscillator. I think the more standard oscillators like RSI, Stochastic and MACD are more useful for daily and weekly charts. I suggest you stick to the Coppock for monthly data only and also only for mass market indices, which is what it was designed for.
There is a formula for Metastock in Newsletter No 1 on my web site. I believe that it is correct. Note that you use the last close of the month for the calculation. Also that it calls for a weighted moving average, not an exponential moving average. This would be why you are getting a different result to my spreadsheet.
Finally, the Coppock signal is when it turns up from below zero, not when it crosses up through the zero line.