Sorry it took me so long to get back to you.
So you are close, but I should have explained this better.
For this example lets assume you have 5 line items and the minimum record is 21 and the max is 25
You will want to summarize the "Minimum" record ID#, not the total.
> [Minimum Line Record ID#]
Then pass the newly summarized minimum record ID# as a lookup field back dow to your line items table.
> [Proposal - Minimum Line Record ID#]
This way all 5 of your line items will have the number 21 in that new lookup field.
Make a formula-numeric field, called "Line #" on the line items table.
>
Use the formula:
[Record ID#] - [Proposal - Minimum Line Record ID#] + 1
That will take the current record, say 23, minus 21, and add 1, thus it will be line # 3.
25-21+1=5, etc.
***Keep in mind that this will only stay true if your line items are made or imported sequentially, thus sequential Record ID's ***
If you have issues with this you might have to create a manual Line # override.
Let me know how it goes.