There is no fixed minimum. The test is whether the committed amount is substantial relative to the total cost of establishing this specific enterprise, and enough to make it operational. Funds still sitting in a family savings account are not invested. Total net worth is context; committed, at-risk capital is what the case rests on.
Price the business, then divide the savings
Work from a costed startup budget: premises, build-out, equipment, inventory, licensing, insurance, payroll and several months of operating expenses. That total gives the denominator the substantiality question needs. Then decide, as a household, what stays untouched for living costs.
If the two numbers cannot both be satisfied, the honest options are a smaller enterprise, additional funding, or waiting — not describing a reserve as though it were invested capital. The figure that most often proves unusable is a percentage quoted from someone else's approved case. It travelled from a different business, a different total cost and a different funding structure, and it says nothing about this enterprise.
Build the denominator from the costed budget above, then ask what remains genuinely uncommitted. Hypothetical example: a dietetics practice with modest equipment needs has a low total cost, so a sum that would look small against a manufacturing purchase can sit high against this one — the ratio moves because the business is different, not because the family is wealthier.