Hi, 

if you would like to set a rounding method for cash, you can follow the next steps:

Go to Point of Sale ‣ Configuration ‣ Settings and enable Cash Rounding, then click on Save.

Go to Point of Sale ‣ Configuration ‣ Point of Sale, open the point of sale you want to configure, and enable the Cash Rounding option.

More info is available on:

https://www.odoo.com/documentation/15.0/applications/sales/point_of_sale/shop/cash_rounding.html?highlight=rounding

Or use the solution within Monetary fields as proposed by Haresh Kansara:
“Hello Odooers,

I think if you are dealing with Monetary fields, then you dont need any code change, only need to set proper

"Decimal Places" in currency settings.

Steps to change Subtotal fields (Monetary) decimal accuracy from 2 digit to 3 digits:

1. Activate Multi Currency from General Settings (Settings > General Settings > Accounting ('Invoicing' If Community)), Now activate Multi-Currencies from Currencies section.

2. Now go to Accounting ('Invoicing' If Community) > Configuration > Accounting > Currencies.

3. In currency form change value 0.010000 to 0.001000 in Rounding Factor field and save record.

I hope this will helpful.

--
Regards
Haresh Kansara
Odoo Application Engineer”