You could use a regex validation on that, a short googling or reading of
this[
^] should get you into it. With a regex matching you could easily implement your check and make sure only numbers are put in.
On the other hand you could check by your self by validating the inserted string with substring or contains.
For example check if the string is longer than 8 chars and then check if the string contains a '.' at the 8th (9th) place.