The tier and the declared value essentially go hand in hand with anything modern. I would recommend selecting the tier you feel the book is worth and declare the value you will be insuring the book for, both going to, waiting at and coming back from CGC.
So as your example, ASM 361, if you are comfortable with the Modern tier, max $200 declared value, then select that and create the submission form for all three. Now if you sent in an Amazing Fantasy 15 in the modern category somehow, then CGC would alter your submission to the proper tier and up charge you appropriately. Otherwise you would be charged for the Modern tier on each of the ASM 361 books you sent, no matter if they come back 9.2, 9.4, 9.8, or even 10.0.
Declared value is important though, in the event something was to happen to the book while in the hands of CGC, that value you specified is what they would cover. So if you feel the ASM 361 is worth $1k, then I would recommend selecting a tier that allows for $1k in declared value.