Here is a link to the level i use.
http://www.ctkprecision.com/all-purpose-gun-level.aspx
You have to make sure you use it on the upper receiver, not the handguard as the handguard may not be in perfect alignment with the upper receiver. The upper receiver is what you need to keep level while adjusting your scope to bring the croshairs of the scope in to perfect alignment with the string that is hanging and which is already perfectly level due to gravity. And if you use this one, take your time with it, make sure you keep it flat, because since it is kind of long, and can have tension on it if you use the elastic band, it can be easy for it to lean one way or the other. The tool is good though because it is perfectly flat and true and it is also thin so it can fit well under and behind or in front of a scope where there might not be much rail space left to rest it on because of the scope mount taking up a lot of the surface area.
When setting eye relief, you want to be in a natural shooting position, not stretching your neck out towards the scope or anything like that.
If you put your eye too close to the scope you will see the image through the scope but there will also be a black ring or dead zone that you see surounding the image. You don't want to be seeing that. But as you pull your eye away from the scope that black ring will shrink so that all you see is the image through the scope, and if you keep getting further away you will start to see less and less of the edges of the image shrinking your field of view. So the key is to set the scope at a distance where you don't see the black ring, but you do see the full image when you are in a natural position on the rifle with the stock adjusted to your prefered length and also at the magnification setting that you plan to use most.
I think a lot of people set up their scopes for proper eye relief at max zoom. I set mine for proper eye relief at minimum zoom and then if i use maximum zoom i adjust my position on the rifle or the rifle its self to give me proper eye relief, because i figure if I'm using max zoom i will be taking a bit of extra time to make the shot, and in theory, i could use that time to make adjustments to the position of my body or the rifle to acquire proper eye relief.
Others might argue that you should have it the other way around so you are in a more natural position when at max zoom so you are more relaxed and able to make an accurate shot. But i feel like i would prefer to have the full field of view in my natural shooting position to be at minimum zoom to aid in quicker target acquisition for closer or moving targets when in a standing or unsupported position.
Also, by increasing zoom, eye relief is reduced requiring your eye to be closer to the scope, and i feel like it is easier to lean in to the scope for a long shot than it is to lean away from the scope for a close shot. Also, your head tends to be more forward on the rifle when sitting at a bench or in a prone position which is when you would probably be using more magnification, and your head is further back or more upright when standing and holding a rifle so that your eye is likely further away from the scope when you would be more likely to have it on a minimum magnification setting for unsupported or close range shots.
All that is why i like to set my scopes for proper eye relief at minimum zoom with myself in a natural "fighting stance" standing position.
Hope that helps