How much does calcium reduce iron absorption?
In short: Calcium can inhibit absorption of iron, but this does not necessary have any significant effect on the long-term iron status in the human body.

Iron (Linus Pauling Institute):


  The presence of calcium decreases iron absorption from both nonheme
  (i.e., most supplements and food sources other than meat, poultry, and
  seafood) and heme sources (meat). However, calcium
  supplementation up to 12 weeks has not been found to change iron
  nutritional status, probably due to a compensatory increase in iron
  absorption. Individuals taking iron supplements should take them two
  hours apart from calcium-rich food or supplements to maximize iron
  absorption.


In various studies, calcium from food or supplements in usual amounts decreased iron absorption by about 50%, but the calcium dose/iron absorption relationship cannot be simply quantified, because it is affected by several factors, for example:


Phytates in food inhibit the absorption of both calcium and iron (BiomedCentral).
Calcium may inhibit the absorption of nonheme iron from plant foods less than heme iron from meat (academic.oup.com).
Different calcium supplements may differently affect the absorption of different iron supplements.




The inhibitory effect of calcium on iron absorption is not linear and one may not be able to express it as a simple formula. Below are some examples from studies.


A review of studies, American Journal of Clinical Nutrition, 2010



  The reported dose-effect relation between the amount of calcium given
  and the degree of inhibition of iron absorption differs from other
  factors influencing iron absorption.
  This dose-effect pitfall is illustrated by 2 studies...[Read more in the linked article ].



A review of studies, PubMed, 2010



  The quantitative effect, although dose dependent, was modified by the
  form in which Ca was administered and by other dietary constituents
  (such as phosphate, phytate and ascorbic acid) known to affect Fe
  bioavailability. The results of most multiple-meal human studies
  suggest that Ca supplementation will have only a small effect on Fe
  absorption unless habitual Ca consumption is very low.



A study, PubMed, 1991 



  When taken without food, calcium carbonate did not inhibit the
  absorption of ferrous sulphate with doses of either 300 mg Ca and 37
  mg Fe or 600 mg Ca and 18 mg Fe. However, at the latter levels,
  calcium citrate and calcium phosphate reduced iron absorption
  significantly by 49% and 62%, respectively. All calcium supplements
  inhibited absorption of the iron supplement when taken with food. The
  absorption of dietary nonheme iron was also inhibited by all three
  supplements. This inhibition was less pronounced from a meal of high
  iron availability and low calcium content (28%) than from a breakfast
  meal of low iron availability and high calcium content (55%).



A study, American Journal of Clinical Nutrition, 2005



  Calcium supplementation reduced heme and total iron without
  significantly affecting nonheme-iron absorption, regardless of meal
  bioavailability.