Hi, 

you could create a field in your model and create a compute method to do this, but you need it in python:

class MyModel(models.Model):
    _name = 'my.model'

    field1 = fields.Float(string='Field 1')
    field2 = fields.Float(string='Field 2')
    computed_field = fields.Float(string='Computed Field', compute='_compute_computed_field')

    @api.depends('field1', 'field2')
    def _compute_computed_field(self):
        for record in self:
            record.computed_field = record.field1 + record.field2

I hope these code help to you

Bye