I don't know if this will help as it's not a particular rule of thumb but...
Nothing wrong with drawing out the triangles and converting the units. I would guess that for your first example, where there are 5 variables, of which there are presumably four given and one unknown, it's unlikely that there is a rule of thumb. They normally only work when certain things remain constant, not when everything is variable, so for these I reckon you'll just have to work them out.
Your second example you are much more likely to get a rule of thumb for, as the 3 degrees component stays constant.
At 3 degrees, for every 2 miles there is a 545ft drop.
At 5 degrees, for every 2 miles there is a 10416ft drop.
At 10 degrees, for every 2 miles there is a 1836ft drop.
These were worked out by trigonometry, using
tan a = opposite/adjacent
with the adjacent being the distance (in feet, 1mile is 5208ft), and the opposite being the height difference.
Conclusion - trigonometry is probably the area to be looking at.
Any good for you?
Edit - FS wrote at the same time that I was, and is making a lot more sense than I am!
Last edited by 4015; 21st Jun 2011 at 11:26.