Goto Technical Settings -> Database Structure -> Decimal Accuracy and change the digits of related precision. This will change all decimal precisions used in your instance. (Which is used while defining fields.). So better to define new decimal precision and add it with your related fields.

You can use attribute digits while defining fields. eg:

discount = fields.Float(..... , digits=5)

or 

define precision as XML data and reuse it whenever needed.

<record forcecreate="True" id="decimal_payroll_rate" model="decimal.precision">
    <field name="name">Payroll Rate</field>
    <field name="digits">4</field>
</record>

rate = fields.Float(...... , digits=dp.get_precision('Payroll Rate'))