Hi,

    Try using api.onchange

In py, 

    @api.onchange('boolean_field')

    def onchange_boolean_field(self):

        if self.boolean_field:

            self.float_field = 1     //Here i have set default value to 1 when you tick the boolean field 

        else:    

            self.float_field = 0.00

In xml,

<fields name="boolean_field"/>

<fields name="float_field" attrs="{'readonly': [('boolean_field', '!=', True)]}"/>   // Here if boolean field is not ticked  then zero is set and float field is made readonly

Hope it helps,

Thanks!!