Firstly, there are different enbeffectprepass.fx files out there which offer different code for DOF settings. Personally I haven't used ENB for a while now, but the ENB that's currently in my Skyrim folder uses a modified enbeffectprepass.fx file from Matsos code, so that's my reference....
The DOF example pictures you've posted are a very good way of helping discuss DOF techniques. Long story short, within the enbeffectprepass.fx file I adjust all 5 main DOF parameters to get the specific effect I want, but your question regarding the tower and grass is mainly due to the values set in Focus Bias and DOF Bias.
Focus Bias will let you adjust the distance in which the focal point / most prominent DOF effect is from the player / camera.
DOF Bias will let you adjust the distance in which the DOF effect starts from, away from the player / camera.
Where you go from here is purely a matter of personal taste and preference......
If you want the DOF effect to not be so close to you (as you've stated) then either push the effect starting distance back, or adjust the focal point to being further back. The reason why I state that this is a moment of choice really, is that (using your DOF distance pictures) some people like not only "Near" to be clear of DOF, but also "Far" and maybe "Farther" and even "Farthest" as being DOF-free zones, but personally speaking I think it more natural-ish and better to have DOF set like your first example where DOF effectively starts at the player, is the most prominent at the "Farthest" and has a LINEAR level of effect imbetween.
You can also set DOF like how you asked, so that it becomes most prominent at just "Far" and remains the same effect after this distance. Again, this is achieved by experimenting with the Focus Bias - using this parameter effectively treats the focal point as being the distance in which the increase of the DOF effect will stop. So for your sake, set this value to a number in which obtains the DOF progression cutoff point at the "Far" distance.
I can't state actual numbers for you to provide, because the different combinations of DOF settings result in many different ways in which DOF can be written. I will provide 2 suggestions though...
1) So far, Static DOF has been covered, but if you are using an ENB which also has Dynamic DOF in it, you may find that you set the Static DOF so that it looks great but once you start moving around and focusing on objects, then you may start to notice that your settings need more tweaking (if it over-blurs when looking at close objects / people etc.).
2) I find that the best thing to do is experiment with DOF settings live (while Skyrim is running). Just open your enbeffectprepass.fx file, run Skyrim in windowed mode, get to a place in-game which is good for setting up DOF, and simply Alt+Tab between modifying and saving your enbeffectprepass.fx experimental value(s) / file and Skyrim itself (you must press "Backspace" in-game to reload your ENB settings. It's an invaluable way of experimenting and within a short while you'll probably get a good understanding of how it all works.
Sorry if I've been too vague, but as I said, this is really a matter for experimentation and preference. Good luck and ask away if you want further assistance!