This is what I have learned about Vit D levels and why they may vary from person to person.

1) As we age, we may not make as much Vit D from sun exposure as when we were young. Some references say this happens. If you take a drug to lower cholesterol this may also impede Vit D synthesis, since the substance in the skin is made from cholesterol.

People in developing countries, have more melanin in their skins to block UV...so they need far more light to make Vit D. Also their diets can be very poor, and they may not get enough calcium from food.

2) In people with more fat on their bodies, the Vit D may go there for storage, and not be available in the blood for doing its various jobs. I have seen reports that overweight people may need higher doses.

I would start at 5000 IU daily. If you have say a blood test showing you are at 20...that would put you at 70 if you utilize it properly. I think 8000 is high, unless your test was in the single digits? Current medical thinking, is that if you have NO test yet, 2000IU is safe enough. The average ratio is 1000IU to 10ng/ level in test. But this varies from person to person.

Then you should get tested again in 3-4 months to see what your calcium levels are then, and your D levels. The risk of hypercalcemia is considered low these days with the new information, but it may still happen. For this reason, some doctors are keeping calcium supplements lower in dose...the link on my Vit D thread at Vitamin forum, suggests 600mg tops now. If you still are taking that dilitazem, I'd watch for leg cramps etc. That is my signal to take calcium now and then, because diltiazem affects calcium levels to some extent, lowering them. Otherwise I don't take calcium at all, normally, because I eat cheese and yogurt and good diet. Yogurt is a good source of calcium.
(for example, two summers ago, when we did not have the big refrigerator on vacation, we couldn't keep food very well. So my yogurt and most of the cheese was out of my diet. I was taking the diltiazem every day there, with a lower dose on my other BP med, and within 2 weeks I was having leg cramps. At home when I have my yogurt every day, I rarely get the leg cramps with the diltiazem at all. So that one change was significant for me!)