@Dave Godfrey I can tell you how I do it.
I put together a line item rehab budget. labor and materials so any overages can be tracked. Once a week I reconcile all rehab expenses into a spreadsheet, which is sent to the JV partner along with any receipts, invoices, photos, and a general update of the project. Most of the JV partners I work with are from out of state, so they rely on me to keep them well updated. Any surprises in the rehab (there's always something unexpected that comes up) are discussed and adjustments to the budget are made.
It's important to have a JV agreement or some type of documentation that clarifies the responsibilities of each party and how profits are to be split. We calculate profits after all rehab expenses and soft costs are taken out.
There are benefits to doing a JV deal rather than just hiring a GC. As a JV partner I manage the entire rehab, maintain and secure the property, handle showings, inspections, appraisals, basically anything that needs to happen locally with the property. I also have a strong motivation to complete the project on budget and on time to maximize my profit. Just something to consider.