"Prices in Credits as well as Fusion Coin prices for tables with other than 5 or 10 levels are not extrapolated. In this case, the sum is built from the existing values without the missing ones."'
When I wrote the template, I already had all vehicles with less than 5 pro upgrade levels at max pro so I could never verify if the progression works here, too. That's why you have to fill in the values manually.
The 15 is, well, a real bug because of my laziness. I thought: "Hey, now that all vehicles have the same upgrade system, I don't have to build the sum of 500 parameters any more, it's always 15!" So I hard wired it. Which was a mistake...
Darn, I'll have to refill it with all the parameters! :D
Yes, I have the SLS Electric (2 levels) at MAX-0000 and the Scion FR-S (3 levels) at 1111-0010. I can only confirm that the first levels need only 1 Common Part. I cannot scroll to Drivetrain for the FR-S because it's not MAXed. But it seems logical that they applied the same reasoning, so it only needs 2 cards for level 2, then 3 cards for level 3.
I'm not sure I understood your point about the sum vs real parameters. EDIT: I understood now. Yes, I'm sure you can use the sum.
Ask me if you need some information that I can give.