Nassau lawmakers on Monday approved a controversial measure allowing the county to receive a lower-than-typical share of revenue from Glen Cove’s planned waterfront redevelopment.

The Republican-controlled county legislature, after hours of debate, voted 13-5 — joined by two Democrats — in favor of the proposal, which allocates $21.3 million from the Garvies Point project to the county over 40 years.

That equals 6.4 percent of the total payments in lieu of taxes from the development, which is slated to include 1,110 residential units, stores, offices and marinas. Nassau would typically receive 7.7 percent of such revenue, said County Executive Edward Mangano’s administration — though opponents of the measure assert it could be far higher.

The difference between the 6.4 percent allocation and what the county normally would receive would go to the City of Glen Cove, which requested similar concessions from the school and library districts.

Discussion on the measure brought out about two dozen opponents of the project. Many expressed concern with the development’s tax breaks, building heights and traffic and environmental impacts, as well as the dispute on the exact level of the revenue share at stake.

“I have a master’s in finance and I’m baffled by the numbers presented,” said Glen Cove resident Marsha Silverman. “The numbers keep changing.”

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Democratic legislators had raised questions about the typical revenue share cited by the county, noting that the varying property types to be included in Garvies Point have different assessment rates.

They seized on the county assessor’s office acknowledging Monday that its estimate of Nassau’s typical revenue share, which mirrored Glen Cove’s findings, was “something very, very cursory,” done on short notice.

“I don’t know how anyone could vote on this,” said Legis. Delia DeRiggi-Whitton (D-Glen Cove).

But backers of the project said opponents were missing the point. They noted that Garvies Point — whose developer, RXR Glen Isle Partners, was granted hundreds of millions of dollars in tax breaks by the Glen Cove Industrial Development Agency — would generate revenue for a long-vacant site now bringing in nothing.

“We’re asking the county to take a little bit less, that’s it,” said Glen Cove Mayor Reginald Spinello. “I think this is becoming something bigger than the whole project.”

Also Monday, the legislature approved an amendment to the capital budget that redirects already available state grant funding to pay for the final phase of reconstructing West Shore Road, a vital North Shore artery that was damaged during superstorm Sandy.

The amendment provides $7 million, previously earmarked for general road resurfacing, for the last portion of the West Shore reconstruction.

The transfer was made necessary, Republicans said, after Democratic legislators refused to provide the needed 13th vote to approve county borrowing to pay for the work.