My advice on the difficulty to shoot for would be try to find something that is slightly above what you think you can do and 'learn up' to the problem. So you may not know how to do it at first but take some time and research, look at examples, etc, and then try to build it.
Your issue would essentially be plugging values into a equation. It would also depend on how complex you want to make it, you can take many things into account such as temperature, runway conditions, etc.
Everyone does learn differently but for me I learn by doing, so the best way for me to learn is to go for som