Well, I started with a 15:1 mix to do a tarp that was leaking due to some abraision. I use it under the sleeping bag in lean-to's. I was afraid I would not have enough to do the floor of the tent, so I added "some" mineral spirits. How much was "some", I don't know. I am guessing about 20:1? I did the bottom (under the tent) then turned it inside out and did inside. Then I turned it inside out, again, and hung it, being carefull to keep the floor and canopy seperated with some scraps from canoe building...I seem to remember two or three days it was out drying/airing out.
Thin "like water" maybe a little less...quite a bit thinner than olive oil. It seemed to cut the sliperiness a lot and sealed the minor leaks at the same time. At least it never leaked, again. My tents don't get a lot of use, only when I get the wife out. The past several years, the wife cannot get out hiking, so, I usually canoe with with her (planning on short portages.) It seemed to really straighten out the "slip to the bottom" you get on most hillocks. Some, of course, I find my feet pressing on the front door before the pad slips down.
Note that this was worked in. The fabric, more like PU coated light weight nylon, doesn't have ripstop "squares" in it so I just went up and down, then side-to-side. The mineral spirits seemed to dry almost as quickly as I was working. The bottom showed some streaks from the treatment but generally it was wiped down fairly evenly. One edge, where I had started,was pretty "dry" before I was started on the inside. I simply turned it inside out to do the other side. It dried for half an hour, then I reversed it again and hung it. The upper silnylon canopy didn't need doing till a couple years ago.