There are little sliders on the f4 screen that allow you to set the projected buy and sell success level. The default is 100% each way but I find something around 75% for sales and 50% for purchases to be more reasonable.
You still won't get an exact number for the next turn.
Another way to judge what is going to happen is to hover your pointer over the top line and the tooltip will show you the last so many turns (10 maybe?) and you can tell what the trend line has been.
You can also look at the market for any given commodity in the Balance screen (hotkey "b") and hover over the commodity and you can see how many of that commodity were offered and how many requested last turn. This will give you some hint as to how many of your offering of that item will be bought on the international market. Going to the third line down (?) on the f4 screen for each commodity the tooltip will show you how many will be sold on your domestic market and that calculation is IIRC pretty much certainly correct.
But the bottom line on the f4 screen should be taken with a large amount of salt.