Different sorts of arrows have to have different centres of gravity.  However, typically, for a regular, flighted, 28" arrow, I'd put the balance point at about 1/3 from the tip.  Remember, an arrow with a nock heavier than the tip will fly so erratically as to make it a joke.  Balance point is important, but often it can be intuitively judged.

Yep.
  Flight arrows might have a balance point that is nearer to the middle than most, that kind of thing.  The Chinese developed a whole system of making arrows based on the balance point.  I'll try and find the information soon.

The balance point of an arrow is expressed in FOC %, or how much Forward Of Center in % of the total length of the shaft, the balance point is. Typical hunting / target arrows have a FOC of between 10 and 15%, target arrows used for long range shooting (up to 90m) have a 7 - 9% FOC, while flight arrows are normally at 0% FOC, or even up to several % backwards of the center point.

Most hunting archers (myself included) feel that the more FOC an arrow has, the more stable it is - with high enough a FOC (17%+), one would not even need any fletching to make the arrow fly well. For comparison, low FOC arrows can turn sideways in mid-air if insufficiently fletched. The biggest reason not to use very nose-heavy arrows is that they make the shaft act weaker, requiring thicker / more massive shafting than most would want.

For consistent arrow flight, all arrows must of course have the same FOC. Practice / field points' weight should match the weight of hunting points / broadheads.

Finding 90# / 33" commercial wood shafts can be next to impossible, since almost no one uses such heavy (both in spine and, inevitably, physical weight) ammunition. I wonder what's the need for 90# shafts if your 75# shafts fly straight, as you say. If they have good flight, it is almost guaranteed that a 90# shaft will be too stiff for your bow / shooting style, and not hit where you want it to. What you need, then, is 75# shafts made from stronger materials to hold up under the strain. The vast majority of commercial shafting is made from Port Orford Cedar, a light, weak wood. Norway pine is also much used, and not much better in terms of strength. I would suggest hardwood shafts: maple, ash and hickory have all been used with success. If you can't find these, make your own (I do). Bamboo is another, very tough arrow material - I've hit rocks with some bamboo arrows without damage to shaft! If carefully selected for straightness and spine, even ordinary garden supply bamboo stakes make good field arrows.

Tuukka