Planning Management

Hello,
I want to be able to define an action just 10 days before a milestone. If the load is too large it would have to be 11 days, pulse 12 etc. But I don't wish it to be placed as soon as possible.
I didn't found a little elegant solution : A negative ejours number by defining the action after the milestone.
Example: convening a meeting which must be carried out 10 days before.
Thank you for your help

Thanks for your reply,
I did not understand... As can be seen on the attached catch, the activity is programed as soon as possible.

Thanks in advance for your answer,