Splitting the fee on someone else's contract, 50/50 or something smarter?

Your overage clause is the part that breaks. The moment you keep 100% above their target, the acquisition side starts setting the target low so their half is guaranteed, and you end up negotiating against your own supplier on every deal. I'd rather see 50/50 on the whole fee with a hard clock: full split if it's assigned inside 5 days, drops to 60/40 in your favor after that because the delay is theirs. That prices speed without giving either side a reason to misreport a number.

On $12k spreads the split argument is worth maybe $1,800 either way. Your buyer conversion rate moves more money than the split does.

Tiering on spread size does something you may not want. Under a $10k spread you'd take 35%, which is $3,500 on a $10k deal, and above it you take 50%. That creates a cliff at the boundary where a $9,900 spread pays you $3,465 and a $10,100 spread pays you $5,050. Acquisition partners notice cliffs and price to them. If you want a size gradient, make the rate continuous or set the tier on your own gross for the month rather than per deal.

The deeper issue is that the split can only price things both sides can observe. Days to assignment is observable. Whether you negotiated the buyer up is not, because nobody can see the counterfactual price. That's why overage clauses tend to collapse into arguments. Ledger's clock structure works because the trigger is a date on a document.

What you haven't raised is how the paper actually moves. If you're taking a share of their assignment fee, you're either being paid as a fee off their assignment or you're taking your own assignment position in the chain, and those are different animals for disclosure and for what your state considers brokering unlicensed. Several states have moved to treat marketing a property you don't own as activity requiring a license, and the analysis turns on your specific state's statute and how you advertise, so get it looked at by a real estate attorney in your state before you set the template.

The operational risk in your tiered plan is that a supplier with two dispo partners will send you the easy deals and the other person the hard ones, and your average days-to-assign will look great while your volume stalls.

The observability point is the whole thing. I've been reading assignment paperwork while I study for the license exam and the one line that keeps mattering is who is named as assignor on the document that gets to closing. If your split is paid as a fee off their assignment you're invisible on the settlement statement, which is fine until there's a dispute and you have nothing but a text thread. A short written co-assignment or fee agreement per deal is cheap compared to arguing about $6k.