Hello @anon71559768,
I am not quite sure if I understand why you would have to reduce the hours.
When you have a task with multiple days, Effort will be split evenly over weekdays (skipping weekends). For example, a 10 hour task over 2 days will be 5 hours each day. If the task goes Friday to Monday, it will be 5 hours on Friday, 5 hours on Monday. (More info here https://asana.com/guide/help/premium/workload)
So basically workload allows you to ensure nobody has more tasks than they can handle.
Based on your message I understood it more in the sense of you trying to see estimated hours vs actual hours worked already, unless I am misunderstanding you?
Many use a combination of workload (to ensure the work is balanced between team members) and then also having custom fields with „estimated hours“ and „actual hours or completed hours“ and then when switching to list view it would calculate the sum nicely or you can also create graphs to gain an overview. I have shared more info about this topics recently here in case you are interested:
Let me know if this helps or otherwise I‘d appreciate if you can explain a bit further to gain more clarity 
p.s. in case you have quite some tasks with many subtasks I recommend rather splitting them up into tasks instead as this helps keeping a clear overview and also with the workload (because then you have several smaller tasks that get completed over time and not big tasks that have a high nr of hours assigned due to many subtasks.)
I mean yes the amount of subtasks would show under workload but still if subtasks are assigned to various people and they kind of take part of the workload of the main task, things can get a bit messy.
Here is a great post prepared by Bastien reg this matter: 💡 Subtasks: when to use them, when not to use them