Your calculation should be based on the day that you have scheduled the flow to trigger.
If the start date of the automatic replies is Monday and it has already passed, you will need to update your calculations for the first run to suit today. And then update the flow again to suit the trigger day and time.
If the trigger day has not passed, you can leave it as it is and then it should work without manual intervention.
On your trigger, you should be able to configure the day and time of the week that your flow triggers and runs. I suggest you configure your flow based on that.
In my case, I trigger the flow to run on 9 AM on Monday, so my calculation will be to add 4 days and 8 hours as I would like the replies to start on Friday at 5 PM.
And I would add 6 days and 12 hours as I would like the replies to end on Sunday at 11 PM.
Hope this helps.
Cheers!