What is the formula to create Currency index (8 currencies of equal weighting) in Create Custom Symbol?

 
I would like to create custom synthetic instruments similar to the currency indexes such as USD index, but giving EQUAL weighting to each of the 8 currencies (USD, EUR, GBP, CAD, CHF, JPY, AUD, NZD).  Can someone give a step by step guide to do this, including the formula required?  Thanks in advance.
 
Currency Strength Index (CSI)
Currency Strength Index (CSI)
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Trading the STRONG against the weak ! The Currency Strength Index (CSI) is a technical indicator to chart the strength momentum of the 8 major currencies. (EUR, GBP, AUD, NZD, USD, CAD, CHF, JPY) The indicator is based on mathematical decorrelation of 28 cross currency pairs. It uses the percentage difference in Linear-weighted averaging (LWMA...