I have a lot of trouble with legs, too. But I'll give some insights.
Contrary to how they might look, legs don't come straight down out of a horizontal line. From the little knobbies of the waist down the the lowest point of the crotch, it forms an obtuse, isosceles triangle. If you study yourself in the mirror, you can probably get the angles right. The legs come downward out of the diagonals of that triangle. If you use that as a starting guide, then the upper legs and waist should look about right. But don't forget that a living body isn't made of straight lines, so you'll have to curvify them afterward to make them look natural.
When bent straight upward, the knee tends to reach the clavicle, so you can use this to measure the thigh length. In humans and plantigrade anthros, the shin is about the same length as the thigh, with the foot extending a bit past that. Generally, a character should be able to kick himself squarely in the butt with his heel. For digitigrade characters, it's a bit different; the shin is usually much shorter than normal, and the foot much longer. This gives the character a somewhat taller presence than a plantigrade character.
Since bipedal digitigrades don't exist as of yet in the real world, it's really up to the artist's interpretation to determine height and proportion differences. I've seen some d-grades with almost no lower leg at all, and some with near-human proportions. It's really a matter of taste.
