procede
The number of accidents will go up by 1 as you say. But the number of flights will not increase that much as the Max was flying for only eleven weeks in 2019.
But you are right, the accident rate can not double, it must be less than a doubling, so maybe around 7 or 8 per million flights. Almost the same as the total hull loss bar for the "Aircraft no longer in service" and the 707/727.