Validate a column via list validation

Hello,

in our environment we have created a standard SharePoint list with a text column. This looks so:

Title; OurTextColumn

Test Entry; 100.0

I would like to achieve that the user can only enter numbers in the following format to the column: 1.0 - 999.0

With the following validation I can confirm that there are not more than characters efore the dot:

=(LEN(LEFT([OurTextColumn];FIND(".";[OurTextColumn])-1))<4)

But how can I check that there is not more than one number after the dot? And can I check that the column only contains numbers and the dot? This check must work in datasheet view.

Thanks in advance.