Relative value is typically based on one of two benchmarks:
1) The last "counting" player
2) The waiver wire player
Questions:
i) How many teams in your league?
ii) How many players at each position?
What I usually do is count the "counting players", so a 12-team 9F/6D format "counts" 108F and 72D. (You can also consider including "bench" players in your count, if it is a "daily set" league and your bench players get used.)
Then I look at past full years of NHL stats and find out the point-total of forward #108F and defenseman #72D.
Last year, for example:
F #108
scored 43pts. Thus, if you value Skinner at 60pts, he's a relative +17pt player.
D #72
scored 23pts. Thus, if you value OEL at 45pts, he's a relative +22pt player.
From there, I'll compare trade value players to these amounts.
In this case, OEL > Skinner... which most of us would probably suggest, without even doing the calculation.
And generally, yeah... it's something like a 20pt difference... 60pt forwards ~ 40pt defenseman... in most leagues.
But this is how to do the math. (or... at least, how I do it.)