When you are in this situation, there is no way to know the correct casting distance to reach the 4m hole (where certainly lots of juicy carps are waiting fou your potatoes) as the maps in RF4 have no scale.
In this particular case of the Akhtuba pond I usually used to set my clip at 20m, right after the last lilly leaves.
But, not fully satisfied with this estimation, I decided to determine the exact scale of the Akhtuba map, namely the length/width of those A-1 to J-10 grid squares.
Technique: assuming that, when the clip is set at 50m, the reel delivers exactly 50m of line (it may not be fully correct but, as we are interested in casting and setting clips, it will do just fine as it is so that it works when you cast),
- chose a clear flat space to cast a line; near the floating village to the west is perfect,
- use a rod that is able to cast at 50m, put the heaviest sinker its test can stand and set the clip at 50m (low level players can use less, it just reduces the accuracy of the process),
- make a full cast and make sure you see this little shock in the line showing that the 50 meters of line are out, not more not less,
- close the reel (press "Enter", don't reel in). Make the line straight on the ground by stepping back till you see some stress in it (the effect of pulling on the heavy sinker); then put the rod on the ground. You have now a straight 50m line in front of you.
- open the map, zoom in to the max, and click F12 for a screenshot of your initial position,
- walk (or run) to the lure you cast, stand right on it and make a second screenshot,
- using an image editor (Photoshop for instance), overlay the 2 screenshots; you get an image like below,
- it is now easy to determine the grid scale using the 50 meters between your two standing points as a reference.
For Akhtuba, and only for this map (as they all have a different scale), the side of a grid square is 93m (+/- 1m).
From that it comes that the map covers a total area of 930m x 930m (totally useless info) and that, in the other grid system (the one showing in the bottom right compass going from 1:1 to 200:200), 1 point is equal to 4.65m (930/200).
Now, when you need to determine a distance during a game, do the following: On your monitor and using a small ruler (quite prehistoric technique but faster than measuring on a screenshot), measure the distance you want to know and the length of a grid square.
In the above example of the pond, I measured respectively 37mm and 102mm at max zoom), which means the center of the hole is at 37 / 102 x 93 = 34m (much more than my 20m assumption). Its diameter is about 20m.
If you round up the 93m to 100m, a simple 37/102 gives you an immediate approximative result, sufficient for a casting which is not exactly space technology.
I was so happy with this finding that I returned straight away to visit my juicy carps.
Result: in one hour, with 3 carp rods on NC, I got only 3 burbots and one 18.6kg catfish (that took me several hours to extract with my 14.5kg reel), two species that I had never seen been caught here before ! So I was clearly fishing in the hole and, at least, my calculation technique seems to work fine.
But, and this is not relevant at all, the value of the keepnet was 62 $ilver only while, in the same time, the other anglers around me were catching several metric tons of carps.
Conclusion: I will stick to my 20m casting...