update and store new high values
 

I have an Excel add-in which continually updates the
prices and other date on stocks which I specify. I might
have a worksheet that looks like the following


Symbol MktPrice HighPrice
ABC $12.50 $13.90
MSFT $35.85 $65.20
TEL $18.99 $19.00


The prices in the second column are continually changing
as the data is refreshed.

How can I create a UDF which will replace the value in the
HighPrice column ONLY when the MktPrice is higher?

I know how to do this with a macro, but this requires
someone to run the macro when a new higher value is
seen. I'd like to have this done automatically whenever
the data changes.

Can anyone suggest a way to do this?