[amibroker] Weighted Average

[Ken Close]

What would be the correct way to get a weighted average of three or four
tickers, with the weightings unequal?
 
WtAvg = Wt1 * Price1 + Wt2 * Price2 + Wt3 * Price3.   ??????
 
I am asking because I thought I remember seeing here or on another list that
doing what I wrote above will give an incorrect result.  But I do not
remember what alternative was suggested.
 
Any help?
 
Thanks,
Ken