I'm not sure what the problem is here - both CT 0.8 and CT 1.0 lines are hairlines. Your aircraft will have one probe or the other fitted (or less likely something in between that needs interpolating). Know which type is fitted and then only use which ever hairline is appropriate - ignore the other completely. The dotted line exists for extreme accuracy and can generally be ignored.
If you're using this computer for the ATPL exams use the CT 1.0 hairline not the CT 0.8.
Diverging from the question a bit but - one of the enduring myths is that the UK ATPL exams are 'designed' for the Pooleys CRP5 and that other computers will come up with a different answer (that is not present in the multiple choices) - this is not true. Many years ago when it was UK CAA exams, maybe, but since it has been a Europe wide JAR exam the tolerances of the questions are such that any computer can be used. Besides if used correctly all computers come up with an answer within 2 or 3 kts of the others. If they don't you are doing something wrong.
The usual exam question mistake is when trying to calculate an IAS from a TAS on the Jepp CR. The CR will come up with the exact correct answer but it requires the user to manually incorporate the ram rise temp into the calculation, using a sort of trial and error until it fits. This is easy (with practice and understanding) and takes seconds but seems to confuse people who use the wrong temp and get the wrong answer.
Having used a number of slide rules, the Jepp CR2 and CR3 are by far the best (fastest, most versatile and most capable and also cheap and indestructible). The Pooley's CRP5 is expensive, mathematically clumsy and generally a couple of generations behind the Jepps in terms of 1950's sliderule technology. It's only used because it always has been, so most text books use it for explanations.
The downside of the Jepp CR is that because it can do so much more, there is more to learn when you first get one - but it is worth it and some functions are the same on all circular slide rules.
It's worth investing in the Jeppesen CR manual/workbook BW2 (about £12) and taking the time to work through it. Without it, you are on your own as many ground school instructors only use the CRP5. The Workbook is better written and easier to understand than every ATPL textbook I have ever read and contains a lot more than the free downloadable CR manual. Lots of really useful methods that will make you love the thing rather than hate it. You will also benefit enormously in understanding the subjects and save many valuable minutes in the exams compared to the CRP5.
Last edited by Pelikanpete; 11th Aug 2011 at 16:56.