You can use widget="percentage" in odoo 15 

< field name="my_field" widget="percentage"/>

Dear MonBou,

Please try below code.

<div name="your_field_name">

    <label for="your_field_name"/>

    <field name="your_field_name" class="oe_inline"/> (%%)

</div>

Hope this help for you.

Cheers,

Ankit H Gandhi.

try this one:

<div>
 <field name="my_field" class='oe_inline'/>
<label for="my_field">%%</label>
</div>