How could I do this?

[gordon@xxxxxxxxxxx (Guy Gordon)]

I would like to write a couple of formulae projecting some Metastock
indicators into the future.  For example, I'd like to have the Simple
Moving Average extended to the right as a linear extrapolation.

Now, it's simple enough to do that by dropping a Linear Trendline on
the Moving Average, and extending the Trendline Right.  But then it
just sits there.  Instead, it should adjust itself each day, so that
it always covers the last N days, where N is the length of the MA.

Similarly, I can drop a Raff Regression Channel on the stock price,
and extend it right.  But I'd like to set one up to be a continuous
short-term projection by having it automatically cover the last N
days.  

Could some kind soul give me a clue as to how to write such a formula?

Thanks for any suggestions.