If you're a new Inventor user you might be employing the "eye-balling" approach inadvertently when you leave your sketches under constrained and/or under dimensioned. Of course this is not a good way to create sketches in Inventor (see: Inventor 101: Simple Fully Constrained Sketches), and you should (almost) always strive for a fully dimensioned and constrained sketch.
With these things in mind, I thought I'd share a link that provides a fun way to practice your "eye-balling" skills, and will likely make you more appreciative of your Inventor sketch constraints and dimensions.
The Eyeballing Game from woodgears.ca, prompts you to adjust a set of geometry for a given condition. For instance here the goal is to drag the lower left corner to make a parallelogram. The blue lines show my attempt, the green shows the correct position:
In Inventor I'd simply place parallel constraints to achieve this:
Here was my attempt at setting a point equidistant to the edges of the triangle:
In Inventor I'd create a construction circle and then apply tangent constraints:
Here is my attempt at bisecting an angle:
In Inventor I'd add a symmetric constraint (or add angle dimensions):
Have a go at the The Eyeballing Game and you'll likely come away with an higher appreciation for sketch constraints and dimensions.