Not exactly. It would only need to be maybe 20% because a 20% reduction in hash rate producers on the legacy chain would cause a significant slowdown. While any block with BIP110 compliace would have 100% compliance with the legacy chain. A single block after activation would likely cascade Miner acceptance (or massive orphan risk). The only failure modes for BIP 110 is near zero adoption (already about 10%) or a Successful URSF which has next to no discussion 2 months from activation.