how to make a matrix formula automatically update?

Hi there!
I am not an advanced Excel user and I need your help for something: I am writing an excel worksheet that produces forecasts from given time series.
The worksheet contains a matrix formula which produces the forecasts once the input time series and other parameters are given.
I need a way to make this matrix-formula update and take a wider cell range whenever we update the input time series as more recent data become available...does something come to your mind?
I somewhat have a feeling that I need to record a macro to do that but I don't know how!

Any help will be much appreciated, thanks!!

What is your matrix-formula?

It uses an add-in to produce ARMA estimates so the formula goes like: ={Arma(B6:B26;1;0;true)}
and I would like it to automatically update if cells b27, b28, .... are available in the worksheet (i.e. they are non zero). Any clue?
Thanks

Is there any other data in column B?
Is ARMA an add in you have made? what is the code? What does the calculation do? Is it dependent on the number of cells in the range (i.e. would =ARMA(B6:B28,1,0,true) give the same result as = ARMA(B6:B26,1,0,true) if B27 and B28 where blank?

You could possibly try something along the lines of:
=ARMA(INDIRECT("B6:B" & MIN(IF(B6:B100="",101,ROW(B6:B100)))),1,0,true)

No the issue is that the value is definitely different for ARMA(B6:B28,1,0,true) and ARMA(B6:B26,1,0,true), and especially the add in does not tolerate blanks that's why this all mess is needed!
The add in is not mine, it is the one available at http://www.web-reg.de/arma_addin.html
Unfortunately I don't think the formula you suggested would work, it returns all #ND

Try this instead
=ARMA(INDIRECT("B6:B" & MIN(IF(B6:B100="",ROW(B6:B100)-1,101))),1,0,TRUE).

No sorry, it doesn't work...can you tell me in words what the formula you typed should do?

it looks at cells b6:b100, if they are empty it gives them a value of the row they are located on -1. If they have a value it gives them a value of 101. It then finds the minimum of these numbers, which should be the last row with data (first empty row -1). It then makes a text string of "B6:B" & last row with data, and uses the indirect function to turn this into a useable reference.

I tested that one to double check it was correct, are you entering it as an array formula?

Yes I am entering it as an array formula...the thing is that probably the add-in does not work with this as an argument (does it make sense?)
Did u double check with the add-in too?

No I don't have the add-in.

does this work:
=SUM(INDIRECT("B6:B" & MIN(IF(B6:B100="",ROW(B6:B100)-1,101))))

yes it works...
ok I know why I get the error, the indirect(...) thing returns #na for the cells where there is either 0 or a blank, and the arma add-in does not tolerate #na do you think I can modify the formula to overcome this?
Thanks sooo much

Sorry, I'm a bit lost.

This:
("B6:B" & MIN(IF(B6:B100="",ROW(B6:B100)-1,101)))
Shoudl give you "B6:B16" where 16 would be your last cell with data.

=INDIRECT("B6:B16") would then give you $B$6:$B$16 as a cell reference which you can use as a formula.
If this works:
=SUM(INDIRECT("B6:B" & MIN(IF(B6:B100="",ROW(B6:B100)-1,101))))
then I don't know why ARMA() would not work with this arrangement, or the comment about returning N/A, unless you cannot use an array formula with the ARMA() arguement. In which case, you could set it up with two cells, one (lets say, for example, C1) with =("B6:B" & MIN(IF(B6:B100="",ROW(B6:B100)-1,101))) then in C2: = ARMA(Indirect(C1),1,0,true) would give you the correct answer hopefully.

ok I see what you are saying....
then the problem is different, before entering the arma(...) formula I need always to specify a nx3 interval where n is the number of available data I have!
of course when I enter your (brilliant) formula the add-in messes things up because the I give it a wider interval than it needs (auto-updating),
the problem is that unfortunately I don't know n from the the beginning and I need to set it very wide !!
That's a mess!! if you have an idea on how to solve this do let me know....infinite thanks

n is the number of cells with data in? either
=MIN(IF(B6:B100="",ROW(B6:B100)-1,101))-6
or if you have no other data in column B
=COUNTA(B:B)

yes!!! Thanks so much the counta(b:B) command works, I only need to select an initial cell range with nx3 where n is the maximum possible!!!

you have been amazing yudlugar