Modified Lehman and Double Lehman are labels used for tiered adviser fee calculations. The engagement terms need to specify the actual rates, tiers and calculation method; the label alone does not establish what will be payable.
Check what counts as transaction value and how the formula treats assumed debt, deferred consideration, earnouts and any retained interest. Ask the adviser to explain how those terms apply to the proposed transaction.
Read the formula with any minimum fee, retainer, expenses and post-termination provisions. Confirm whether amounts already paid are credited against another charge and when each payment becomes due.
For a broader engagement checklist, see comparing adviser proposals.