Comment #2 of 2 on 3-26-2019 <<< I should have added for purposes of saving lives*** [via public education, and not with reference to merely the empirical origins of the 7/10 Rule] that your alpha- or beta- or gamma-measuring equipment is only as good as its calibration, and it needs to be checked as to its calibration periodically-- like any other type of measuring instrument. Especially anything that can determine whether you live or die. Since you particularly want your radiation meters to be accurate at 500-1000 Roentgens per hour (500 Roentgens/hr is usually fatal with an hour's exposure, 300 R/hr for an hour kills 50% of exposed humans) you want your meter checked against a source (typically cesium-137) that delivers up to or beyond 500 R/hr. And you need to make sure it is accurate in the microR, milliR, 1 R, 10R, 100R, 200R, 300R, and 400 Roentgen ranges since your survival depends on accurate measurements and summations of your various exposures over time, e.g. if you have to climb out of your bomb shelter and forage for food. If you can get to a safe place and stay there for 2 days, the radiation levels will probably have come down to much safer levels, viz. they should decrease by a factor of 100 over 49 hours. But this is only a flimsy guess and it involves the fact your life is at stake. Not a time to actually be relying on guesswork when there is a better alternative.
At any instant, if you can get 6 inches of concrete between you and the radiation, your exposure will be cut by a factor of approximately two, to half the original. If you can get 45 inches (115 centimeters) of ordinary dirt between you and the radiation, your exposure will be cut by a factor of something like 1,000, e.g. from 1,000 Roentgens/hr down to only one R/hr. These estimates are approximations only, and each radiation situation is different, e.g. dependent on each unique isotope variety, quantity, and in other words percentage in the mix. Most fallout looks like weirdly shaped irregular brown glassy sand or gravel with rounded edges, since it is largely made up of melted soil that got tossed by the blast, congealed quickly, and fell from high in the sky. Weird sand is easy to brush off your clothes or comb out of your hair. Other fallout consists of solidified vapor from nuclear bomb silicate mist, and it is totally invisible at the size of micro- or nano-particles. This is extremely hazardous as to alpha radiation to the lungs if inhaled, so again the measurement for alpha (in the dust on your face mask) can be =====SUPER= CRITICAL====== for avoiding the Darwin Award. Ditto for alpha dust getting detected or undetected down your throat. So, an accurately calibrated radiation-reading instrument is the best recourse. And, overnight when you are asleep, another dose of fallout may have blown in from another bomb detonation or washed out of the clouds with ordinary rain. So, accurate measurement is the only way to know how safe you might be.
***The real bottom-line answer to this question is: The purpose of the 7/10 Rule is to be a simple generalized empirical rule of thumb to help estimate your radiation risk. Since it is only an approximation, it can be deceptive, e.g. like where to stand if a semi tractor-trailer is barreling down the road and coming your way. By the rule of thumb, you would expect that you should stand on the shoulder to avoid the oncoming semi. However, if the circumstances are peculiar and the driver is drifting onto the shoulder, you better not be following the rule. Rather, you had best be observant of this particular danger at this particular time at this particular place. That's why if you follow the empirically-derived 7/10 rule you have a better chance to stay alive than if you hold a wet finger to the wind. But, better than betting your life on 7/10 it's an enhancement toward avoiding the Darwin Award if you have competent equipment, have competent calibration, and have competence in understanding what the numbers mean.
Currently, in a heightened sense of dread owing to questions about North Korea and Iran, various formerly available economical and efficient radiation measurement equipment is being commandeered by civilian government employees and military government employees, and is no longer available to ordinary citizens at any price. What is left for us peasants is largely bulky tedious junk equipment the government doesn't want to rely on. This policy of government commandeering first and citizens lives last was formulated under Obama and it is continued under Trump. Be advised you are on your own, and if you don't learn to fend for yourself it's likely that Big Brother won't be there when you need help, since you are expendable. Similarly, there is no longer a coherent organized large scale grain reserve for the United States. Since 2008 it has held cash rather than food https://en.wikipedia.org/wiki/Bill_Emerson_Humanitarian_Trust.