Bundles and discounts when owning one item - buy the cheaper item first or more expensive?

[Peakvcty]

When buying a bundle when you already own one of the items, how does the discount work in terms of how can you know how much of a discount you would get towards the bundle, the price you paid for the app or its current price?  Also if the item is on sale how does that affect it?  Anyone figure this out?  Ill give an example:

Right now Demeo is on sale for 20.99 vs 39.99 discount of 47.5%
Demeo Battles is also on sale for 11.99 vs 19.99 discount of 40%

The bundle is of both is 34.99 vs 59.98 so a discount of about 41.6%
In this case its actually cheaper to buy then separately on sale 20.99+11.99 = 32.98

What I am trying to figure out is how do they determine what credit you get toward the bundle? What you paid, the current price of the item? Would it be cheaper to buy one first over the other, such as buying demeo battles, then the bundle or buy demeo and then the bundle? 
I own walkabout mini golf and its not consistent the credit given for it on the 3 bundles, the game is 14.99 but if you look at the 2024 XR-Lympics bundle it gives you a 9.95 credit for owning walkabout, but on the Lost Cities bundle you get a credit of 5.91, and then looking at the Whole In One Bundle - on my account is shows a price of 66.83 before credit and a price of 47.83 meaning $19 credit for owning walkabout already.  If I log out and check the price is actually 56.99 for the bundle so a credit of 9.16 for walkabout.  

I went ahead and bought demeo first for 20.99 and now the bundle is showing 6.99 vs if I bought it seperately on sale for 11.99.  I wonder if I had bought demeo battles for 11.99 how much the bundle would have been compared to the sale price of 20.99.   buy the items separately would have saved me $2 over the bundle on paper but I actually saved 7.01 buy buying Demeo first and then the bundle getting both for a total of 27.98 vs 34.99 or 32.99.  Demeo had a larger discount, so I took a chance that might give me a bigger credit, but not sure if I had done it the other way i might have gotten it for even less than 27.98.