PCGS Set Registry Question

This question pertains to how the "set rating", and thus rank, is calculated for a PCGS set registry. I am considering using my slabbed type set, but if I am understanding their formula correctly, I could lower my score by adding lower graded tough coins. Let me explain;

The weighted GPA is calculated using different coin ratings, where some coins are worth more than others, as well as several grade deductions or bonuses. I'm fairly certain I understand this formula. The weighted GPA is only calculated using the coins one already has.

The percentage complete is simply a percentage of how many coins you have compared to how many are available.

Then the set rating is calculated by multiplying the two.

For a type set, I don't have my bust dollar yet. If I add one, I'm fairly certain my rating will actually go down unless I get the bust dollar in a high grade.

Take this theoretical example;

Let's say a collection includes 10 coins, 9 common and one super key date. Let's say the 9 commons all have weight of '1' and the key date has a weight of '10'. Let's also say there are no deductions or bonuses here.

Now, if I only have all 9 common, all at a grade of MS60, in theory, My weighted GPA will be 60 and my percent complete is .90. My set rating is thus .54.

Now say I add that one super key, but could only get it at G-4. My percent complete is obviously 100% now. My weighted GPA is now 37.6 because the really tough coin was only a G-4. My set rating will drop all the way down to 37.6 too.

That was an extreme example, but in reality, I don't want to buy an AU bust dollar. I'm fairly certain my set will be decreased by only adding a $2000 bust dollar to my type set.

This is stupid, am I missing something here.