A perfect +35% FCR on Spirit (in addition to Magefist gloves) will put you at the 105 FCR breakpoint, which would mean you don't need to have any FCR on your rings (and can wear Nagelrings, +Mana rare rings, SoJ's, or whatever else you prefer).
Likewise, without the perfect FCR on Spirit, you're always required to have one FCR ring to reach that breakpoint (irregardless of the roll). Because of this, I would keep pursuing a perfect FCR rating on the Spirit.
But, sometimes the base shields are easier to get than the Hels. If so, you might keep the one you have and pick another shield to re-roll. If it's better, reroll the original +30 until you get the +35 shield. Then you've got a spare you can trade away pretty easily too (even if non-perfects are cheap, they're still worth something).