Most opposing expert reports don't fail on arithmetic—they fail on choices. Two choices deserve scrutiny in nearly every contribution or valuation report you'll see.
The start date. Where the analysis begins is rarely neutral. An expert who starts the ledger at a refinance rather than the acquisition erases years of one party's capital. Ask why that date was chosen, what happens to the ratio under the alternative, and whether the report even discloses the sensitivity. An honest report shows both; a results-driven one shows one and hopes nobody asks.
Asymmetric treatment. Watch for categories credited to one party but not the other under the same logic: carrying costs counted when Party B paid them but treated as "use and occupancy offset" when Party A did; improvements capitalized on one side, expensed on the other; imputed rent applied against only one co-owner. Asymmetries are easy to bury in an appendix and devastating to expose in a two-column chart.
A structured rebuttal doesn't need to be long. A contradiction chart—the opposing report's own numbers, restated under consistent assumptions, with the swing quantified line by line—often does more work in cross-examination than twenty pages of narrative.