The explanation makes sense, but I'm afraid that's not the correct way to do it tho :-(   It's introducing a circular dependency, which is why @sofiagvaladze had to split the compute in 15.2[1].    I believe you'll have to do the same thing here    [1] : https://github.com/odoo/odoo/blob/master/addons/hr_expense/models/hr_expense.py#L173