How can i conclude my forward current should be this in LED,opto-coupler?
The absolute maximum rating is the value for which the manufacturer guarantees that the chip will not immediately go up in smoke.
During normal operation, you shold stay away from that value; even going near but staying below can accelerate aging if you use it for a long time.

The documentation should show a "recommended" or "suggested" value.

With an optocoupler, the current that it is designed for can be seen in the test conditions of the current transfer ratio (CTR) specification; for the 4N35, this is 10 mA.
(The CTR changes with current. While the 4N35 datasheet shows graphs for that (figures 2–5), those are typical values, so there is no guaranteed range for the CTR if you use any forward current other than 10 mA.)