How do I limit the number of complete sentences in a spreadsheet c

I'm trying to keep the number of complete sentences to the maximum number
within a 400 character limit. I understand how to use LEN to limit the
number of characters, but don't get how to take the resulting cells that have
incomplete sentences and delete the partial sentence at the end.

Any advice would be appreciated.

If cell A1 contains:
This is a test. This is

and cell A2 contains the formula:
=MID(A1,1,FIND(".",A1,1))

The result will be:
This is a test.


--
nbrcrunch