I would create a formula that covers all sheets and switch some of them
on/off. This is =sum(SH1!A1*TRIG1+SH2!A1*TRIG2...)
Now you need to add a table of workbooks SH1, SH2... with accompanying
triggers TRIG1, TRIG2... to have a full control over your formulas.
I have been using this solution for a couple of years, moreover you can
apply additional parameters e.g. change currencies...
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HTH
Tomek Polak, http://vba.blog.onet.pl