Coal Mine Rescue Field Test 5 Taken from the 2018 Kentucky River Mine Rescue Contest held in Hazard, Kentucky Progress Indicator: Question 7 of 10 7.  Marginal calibration occurs if the span reserve is between ___________ of the applied (calibration) gas value/concentration. Choices: 30% – 50% 40% - 60% 50% – 70% 70% –80% None of the above