Prorating Freight charges across lines in a ship set

We have a freight charge modifier that needs to be prorated across the lines on an order that are grouped together in a ship set.

We have a lumpsum freight charge modifier that we apply automatically and it appears to work, but acts inconsistently when lines are added to a ship set.

I would like to get some insight on how to correctly setup this modifier to seamlessly prorate across the lines in a ship set.

-What application method is correct (Lumpsum, amount etc.)

-What pricing phase to use?

-What is the setting that tells the pricing engine that a modifier needs to be prorated across all lines in a ship set? Is this automatic functionality for a freight modifier? Is the prorating always done equally by dividing the lumpsum amount by the number of lines?