The update record server behavior has that column defined as an integer:
$UpdateQuery->bindColumn("MCbmi", "i", "".((isset($_POST["MCbmi"]))?$_POST["MCbmi"]:"") ."", "WA_DEFAULT");
If you change it to double it will allow decimals:
$UpdateQuery->bindColumn("MCbmi", "d", "".((isset($_POST["MCbmi"]))?$_POST["MCbmi"]:"") ."", "WA_DEFAULT");
It may be the same on the insert page.... Also make sure the database column is set as a double instead of integer as well with phpMyAdmin.