I am trying to create an Investment Portfolio Index. which is based on a changes in value from three different asset classes (Cash, Bonds, Shares). Cash makes 40% of the index, Bonds 50% and Shares 10%. I have assumed the Index commences with a notional value of 1000 and that value will change in line with movements of the underlying asset classes.

I have included a table below which shows that the Shares value falls by 50% on 1/7/2014 (the others don't change to keep it simple) and I would like the Index to effectively show the value being 5% lower (950). However if the Shares increase in value back to their starting point I need the Index to accurately show that increase (in this case back to 1000 for simplicity).

I'd appreciate any assistance to get the correct formula as I want to record daily changes to my Index.

Weighting	40%	50%	10%
	Cash	Bonds	Shares	Index
Starting Value	3000	2000	7000	1000
01/07/2014	3000	2000	3500	Formula??
02/07/2014	3000	2000	7000	Formula??