ADDT´s Mask Field widget doesn´t allow for a variable number of digits preceeding the decimal, so I´d suggest to rather use a regular text input field and define a Regular Expression for the corresponding field validation:
This regex will allow a "between one and unlimited times" number of digits preceeding the decimal, but will enforce a decimal followed by exactly 2 digits.
Adobe Community Expert, Dreamweaver
Joe, not to dismiss the power of regular expressions but another alternative would be to configure your field to submit as 'double' and then select the float or positive float validation format for that field. The float/positive float options become available only after you have configured your field to submit as double.