I'm sure there are some "rules of thumb" within the tank designing community, but I'm not aware of any in my work. My only reaction to your images is that your lifting lugs appear to be arranged in such a manner that the welds attaching them to the tank body are loaded in tension. I have had more than one experience in the past with welds loaded in tension failing. One of them resulted in a multi-million dollar lawsuit when a fork truck driver was injured inside a trailer that turned on its side when a weld in a support leg failed. (Fortunately I have not been the designer in any of these cases.)
Because of these experiences I have a gut level instinctive reaction when i see welds loaded in tension. I will always go out of my way in any weldment I design to make sure that any critical welds are arranged so that they are loaded in shear rather than tension.
I can't really tell for sure in the images you provided. The first image shows the lugs on top of the tank, giving the impression they are simply welded to the top. Not good. The second image shows them welded to ribs under the cover sheet, but a cannot tell if they are attached to the tops or the sides of those ribs.
In your case my first choice would be move the lugs to the outer wall of the tank where the vertical force would create a shear load. You can make the attachment plates large enough to spread the load as much as you want.
My second choice would be weld them to the SIDES of the ribs under the cover sheet. Again, make the actual attachment plates as large as you can.
That's all I know. Just add my thoughts to any guidance from the pros.