29 CFR §4204.12

The criterion under this section is that the amount of the bond or escrow does not exceed the lesser of $250,000 or two percent of the average total annual contributions made by all employers to the plan, for the purposes of section 304(b)(3)(A) of ERISA and section 431(b)(3)(A) of the Code, for the three most recent plan years ending before the date of determination.