I would like to ask two things about density altitude:

1. In my calculations to get ISA tempt, I will use 15- 2 x(elevation) as per formula. If the elevation is 2400ft, I will calculate with 15-2x(2.4), however, I noticed the answers in the textbook does not retain the digit after the decimal point, and instead rounds it off. If I use more current method of retaining the digit after the decimal point, is that ok? and a more accurate way of getting my answer? Or should I round off?

2. In the take-off and landing charts, the chart starts off at the pressure height (as indicated by the 'start here'). I once accidentally tried to start off with the density height instead as I had the parameters to calculate it. However, I noticed the lines drawn to get my eventual landing weight with density height differs to when I used the pressure height as the starting point, thus getting the wrong answer with the former method. Why is that so?

Edit: Just realised this should be under Performance. How can I move this topic?