Some responses:
1) A straight average is normally a terrible idea for an overall card grade. Think of a card with two moderate creases leading to a 2/10 surface score. Then give it perfect centering, edges, and corners. The Average is 8/10. That's why BGS has a cap on the difference between the lowest subgrade and the overall grade.
2) Determining how to decrement a card grade's based on flaws that PSA calls qualifiers. Are you going to take the surface grade all the way down to a 2/10 based on writing or are you going to give it a 10 with a qualifier of sorts? Consistency in identifying out-of-registration cards and deciding on the proper level of point loss is also important.
3) You probably want to focus group various flaws to respected collectors *before* announcing your scoring algorithms. Continually tuning them, I would think, would actually hurt you in terms of acceptance and trust.
4) Centering could be graded on something like 2.5 point increments. 52.5/47.5 or better is a 10, 55/45 a 9.5, all the way down to miscut getting a 1. Another point could be taken off for slight tilt or 2 for drastic tilt.
Centering Grade
47.5 52.5 10
45 55 9.5
42.5 57.5 9
40 60 8.5
37.5 62.5 8
35 65 7.5
32.5 67.5 7
30 70 6.5
27.5 72.5 6
25 75 5.5
22.5 77.5 5
20 80 4.5
17.5 82.5 4
15 85 3.5
12.5 87.5 3
10 90 2.5
7.5 92.5 2
5 95 1.5
2.5 97.5 1
0 100 0.5 (Miscut)
5) How much does/should a pinhole count toward the surface grade? A card with a pinhole may be missing 0.1% of the surface area of the card, while one with rounded corners may be missing 5% of the card area. However a card with rounded corrners is still regularly called VG (3/10) and one with a pinholes is currently capped at GD (2/10).
6) How would you declare a hand-cut card like Post Cereal to be "full"? Would it require the full black border or is part okay?
Some things I regularly see requested: specific centering measurements, specific card sizes, noting of hard to see flaws (pinholes, surface wrinkles), specific alterations on an altered card.