How to use arithmetic when setting a variable value in Ansible?

One little thing to add. If you presume the math multiplication has precedence before jinja filter (| sign), you're wrong ;-)

With values like

total_rate: 150
host_ratio: 14 # percentual

"{{ total_rate*host_ratio*0.01|int }}" => 0 because 0.01|int = 0
"{{ (total_rate*host_ratio*0.01)|int) }}" => 21 as one expects

You're really close! I use calculations to set some default java memory sizes, which is similar to what you are doing. Here's an example:

{{ (ansible_memtotal_mb*0.8-700)|int|abs }}

That shows a couple of things- first, it's using jinja math, so do the calculations inside the {{ jinja }}. Second, int and abs do what you'd expect- ensure the result is an unsigned integer.

In your case, the correct code would be:

vars:
  ramsize: "{{ ansible_memtotal_mb * 0.8 }}"