Great question!
I'm no expert, just offering ideas on positioning statements/approaches:
1. "I'd like this position to be the basis for a long period of contribution to the company, so the compensation should be enough to protect both of us against inquiries by other employers." (can be used as a lead-in to your $120k figure)
2. "I want the compensation for this to be low enough so the company knows it's getting value, but high enough to know I'm valued." (can be used just like 1, or as a separate statement later)
3. Perhaps you can talk to HR, a manager you trust, or a trusted veteran who is knowledgeable in order to find out what principles, if any, the company has for setting salary. Sometimes if you ask, there are real answers to this question, and they could shape what you then conclude is a reasonable amount. (Side question: Does your company have cost pressures where the $140k is more than they want to pay in the future? Not intending to discourage, just have seen such situations.)
By the way, Stuart Diamond's "Getting More" is a modern classic on negotiating, if you have time to read it. Could help in other situations too. Anyway, best wishes (and let us know what happens...)