It's looks as though it falls off dramatically because you have multiple lines running on different scales.... it also depends on how timing is being pulled, is it setup linear? Or does each row pull it's own amount?
If you are pulling timing linearly you technically only need one row above 100kpa.
Let's say you wanted to pull 1* per psi up to 15psi. With a wot total of 27* at 100kpa.
Simply set the 100kpa row to 27*
Then make a row above it at 203kpa with timing set to 12*
The boost retard will be a perfectly linear 1*per psi up to 15psi where it will level off to 12. But it will not retard past 12 even if you pummel 30psi into it.
Sometimes fewer rows is better so that you don't play with the number in between them too much. One row above 100kpa with your total retard amount is much simpler and will act more like a BTM. Set it and forget it.