[amibroker] Formula to plot a standardized relative strength?

["personal592002" <personal592002@xxxxxxxxx>]

How to write a formula that will plot a standardized relative strength 
that sets the plot of RelStrength to 100 as of a specified fixed 
date?  This is what I have so far:


multiplier = Ref( RelStrength ("SPY"), -256 ) ; // how to set this to 
a specific fixed date rather than a 256 bar lookback?

StdRelStr = RelStrength ("SPY") * 1/multiplier * 100 ; 

Plot (StdRelStr, "Standardized Relative Strength", colorDarkGreen, 
styleDots);