@John Clark we use a custom one however nothing super crazy. Your analysis and back of the napkin will vary probably quite a bit depending on asset class, location, size etc. But I think @Andrew Johnson rule of thumb of 50% will get you in the general ball park. We deal typically with 100+ units. My recommendation is to start networking with brokers and getting on their lists. You'll typically get the OM, T12, and current RR which will give you enough to get started. Once you've seen a few of them you'll start getting the hang of rent comps, price per sqft, price per unit, what certain expense items should be per unit in your submarket, etc.
Additionally, your proforma most of the time should land somewhere between the actual current financials and the broker proforma. Typically, deals that will work have upside so your performance should be better than how they currently sit and the broker's proforma will always be aggressive.
Good luck!