800IU is a very low dose. It USED to be considered good, but the new information changes that.
The average ratio of units of D3 to blood test results is 1000IU for each 10ng/ of blood test results. So if you know your initial D levels, you can determine how much to take to get you into range.
With all the new research being accepted by many current doctors in teaching facilities, the range is now 50ng-80ng target.
Lab results are reported as 30ng-80ng. (30 is the minimum to prevent rickets).
Here is the new medical video on Vit D.
OTC combo products are not going to have therapeutic levels of anything in them, as a rule. To find therapeutic levels of any nutrient now in US, you have to be informed and look for them mostly on the net.
It is really best to get tested for your D status in order to understand what YOUR BODY really needs. General old levels are not longer thought to be accurate anymore.
Most Calcium in supplements (when D is low or below normal) is not absorbed. Average is about 20-25% per dose. So not all the 600mg is absorbed each pill. When D is optimum more can be expected to be absorbed. Also you have to factor in "elemental values" and the 600mg is not elemental. So it is possible you get less than 100mg from each 600mg tablet.
My magnesium thread on Vitamin forum explains "elemental". Most labels do not give this yet, and so people remain unclear and confused as to what their supplement is really providing.
Calcium carbonate is about 40% calcium (the rest is carbonate).
600mg of calcium carbonate is 240mg of calcium elemental of which only about 25% is absorbed with poor Vit D levels.
That would be 60mg perhaps. With mega doses of Vit D3 in the 5000IU range and above absorption may increase, but studies don't say yet how much. So what looks like a big number isn't really.
Most of the calcium you take in tablets remains in the bowel. In fact, this bowel excretion in the stool is thought to help prevent colon cancer.
edit to add: The goal to achieve ranges given by grassrootshealth.net are 40-60ng/ml target. The ranges from VitaminD council are 50-80ng/ml
However, both sources agree anything in the 30's or below are too low.