Couple of things to bear in mind, your comfortable lead depends on whom you are trading your excess to (obviously). But you should in general try to trade your excess stats to the person who might knock of your competitor in said stat. Since each league uses varying starting positions, a raw number is inappropriate in this kind of question.
I'd add it up (and overestimate a bit) on what would be needed to catch you. It doesn't pay to win by a lot, but leave a little cushion-- just in case. Then figure out what your opponents in your surplus categories have, and try and drive a trade to those teams first. Sometimes, if I think my contribution will knock someone else down, I'll take less overall value.
In general, I find runs average, era and whip tough to make up. BUT, you need to look at tiers too. In some of my leagues, it might be tough to get to the next tier, but if you do, you could be looking at 2-3 points, so you need to bear that in mind.
I think this is the central question that makes roto fun by the way. It's math definitely, but it's also an art. Juggling how much value to extract for dealing surplus is the most fun. If you win by a lot, it makes no sense to you, as you have overextended resources. In that scenario, taking less makes some measure of sense, because you are really "taking nothing" if you win. I'd probably err on winning by 3 percent or so (depending on cat) and working from that.
A fun, but a BIt risky way to do this is if you have a good win lead, and want to hold ground in pitching ratios. If you have undervalued MRs, this is the time to pull them out and go strong with them, while trading, say one stud arm. That can help a lot. Obvious ones to trades are saves and steals, as its both kind of easy to predict a reasonable pace, and its one of the easier ones to make up ground in.
Just really crucnh the numbers, and see what people's individual stat standings are, as well as the economics of player distribution in your league. Knock down your competitors, or at least make your competitors think they have you, then you should be fine. (don't be afraid to trade to a competitor either, it just takes massaging--give them a better closer than they expect -- after crunching numbers), than they expect. Perhaps, through your trade, they would still be 10 steals short, sometimes, its worth it to float, as they might gamble, you get the drift.