There's no single right number, but your own build tells you where a strong stand sits on this slate.
Work from the leverage column rather than a target percentage. Leverage is your exposure minus the projected ownership, so it shows how far over or under the field you are on a play. Sort by it and look at the whole curve, top to bottom.
Then decide where this player belongs on that curve. If he's your strongest read on the slate, you want him near the top of your leverage. If he's one of several plays you like, somewhere in the middle fits better. Set a minimum exposure that moves him there, apply it, and look at the curve again.
Keep an eye on the Saber Score or Sim ROI for your lineup group while you do this. It tells you what each change cost. If it holds steady, the sims are comfortable with the stand. If it drops sharply, the play is more expensive than it looked, which is worth knowing before you decide how far to push.
Any adjustment to exposures is ultimately going to decrease your Sim ROI or Saber Score somewhat because the default lineups are what we think is best to play. It is ultimately up to you to determine how much conviction you have in your take, and how much theoretical Sim ROI or Saber Score you’re willing to sacrifice to exercise it.