Sorting formula

Working on this project, i have issue to formulate this..need help, let's say
Item A has total of 20.
Item B has total of 25.

Point Consideration
1. These two items A&B can be put inside a same big box
2. One big box is filled with 2 small boxes
3. Each small box only can fits with 8 quantities. So total 16 quantities in a big box.
4. However, Item A and Item B cannot fill in the same small box, meaning if small box A has filled 6 quantities, then remaining 2 cannot be filled with item B
5. Scenario: small box with item A is filled with existing solution, small box with item B is the additional in next solution.

I have done the solution for Item A, but i couldn't solve for the new add in Item B. In this solution i need to expect:

Box 1: 16 item A
Box 2: 4 Item A, 8 Item B
Box 3: 16 Item B
Box 4: 1 Item B

You may refer the formula below that i used to output for Item A alone

Box 1 formula: =IF(25/(2*8)>=1,2*8,MOD(25,(2*8)))
Box 2 formula onwards: =IF((25-SUM(G$40:G40))/(2*8)>=1,2*8,MOD(25-SUM(G$40:G40),(2*8)))

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