Plan Aggregation Level best practice

Team,

I will like to know if we have any best practices around how to set the Plan Aggregation.

We have Days/Weeks/Period.

My Observation is that with Monthly aggregation and days and weeks set =0 - do not reflect the Leadtime based date calculations properly.

Example if Item A had 90 days processing leadtime,with a 5 days/2 day off Calender (assigned to Supplier Cal in ASL) the Start Date and Due date range donot honour 90 days - it typically shows certain days lesser.

If I set Day aggregation to 90 days and week to 4 weeks and period = 4,I find that the Start Dates and Due date do calculate reflecting the 90 days processing LT.