Pre-money versus post-money goodwill: what are the modeling-test pitfalls?

The pitfall is mixing the target's pre-deal book equity with the buyer's post-deal purchase price. Goodwill is calculated from acquisition consideration and fair value of identifiable net assets at closing, not from post-deal combined equity. Do not carry forward old target goodwill, do not double count fair-value step-ups, and remember that deferred tax liabilities from step-ups increase goodwill.