It's not a problem you'll ever have in practice, as it will always be obvious to you whether that "2" on your wizz wheel is 0.2, 2, 20 or 200. It is not a problem you'll have with time/endurance etc issues as the inner scale of time is unmistakable.
But lets take some practical examples:
Conversion Nautical to statute miles
- Set 90 next to the "NAUT" marking
- Read the value next to the "STAT" marking. it is somewhere about 10.5.
- You know that nautical miles and statute miles are fairly similar
- You therefore know this cannot be 10 Statute miles, nor 0.1 so must be around 100
- The answer is 103.5
Calculate speed from distance and time
- Distance is 26NM and time is 13 minutes
- Set 13 on the inner scale opposite 26 on the outer scale
- The rate arrow points at the speed. It says 12.
- I'm flying a standard single engine prop. Can it fly at 12 Knots? Can it fly at 1200 Knots?
- The answer can only possibly be 120Knots
Fuel burn rate
- You took 32 gallons of fuel and flew 4 hours 20 minutes
- Set 4:20 on the inner time scale against 32 on the outer scale
- Rate arrow points at 74.
- Did I burn 74 Gallons per hour? My Cessna only holds 40! Did I burn 0.74? That seems awfully low - with that my endurance would be measured in days when I know its roughly 4 hours.
- The answer can clearly only ever be 7.2 Gals/hour burn rate
The thing with being out by a factor of 10, is that it is always obvious you're out by a factor of 10. 2x, 3x even 4x you might get confused. But I don't think you ever will with 10x
The even shorter answer is that once you've gained your PPL you'll only ever pull out your E6B to answer these such questions, and any other time you'll use modern, electronic tools to answer these questions. Slide rules; gone from the classroom in the 1970's but still used to teach people to fly.