Does the oil lever have to be exactly at full on the dipstick ?

If you're in the "area" that's fine. A little over will not hurt anything. Typically the oil dipstick will have an area for proper operating oil level.

Generally, when the level hits the add mark, it is a quart low, and you do not need to add any oil until it gets to this level. However, do not let it go below the add mark level. If you try to keep it at the full level, you'll be putting in oil a few capfuls at a time. I will not mix brands of oil, or oils of different weights. I suggest you stay with one particular brand of oil and whatever weight the car mfg recommends. Personally, because my vehicles rarely use any oil between oil changes, I check the level every week or so. I don't add any oil until the level is between the full level and the add mark, and this is usually about 1/2 of a quart. It is wise to have a couple of quarts of close by - I keep one in each vehicle, and one in the garage for those "just in case" times.

In my opinion it's best to bring the level of the oil just below the top of the "full" mark or the top hole. Oil will expand once it reaches temperature and if you check the dip stick when the engine is hot it will show the level of the oil at the "full" mark. By leaving a little room you won't force the pressure to push the oil out through the valve cover gaskets. This is how I prevent oil spots from showing up on my garage floor.