Oh I didn't realize the tests didn't state the gestational timing- that seems odd to me since the entire obstetric world seems to rely on the 40 week system. Hmmm... are we sure about that? Does it state on the test "weeks since conception?" I was making the assumption that your hCG levels are just lower than the averages built into the test. Please note that a single hCG level means virtually nothing by itself b/c of the great variability- including when implantation actually occurred. It is the doubling time that matters.
At any rate if you do want to avoid an early u/s then you could put conception 24-36 hours after that OPK+ (doesn't seem stupid now does it? ) which would be June 1st or 2nd making you 19 or 20 DPO today. By standard 40 week calculations, that would make you 4 weeks and 4 or 5 days pregnant today and give you a due date of February 22nd or 23rd. I agree with JMJ- I like the latest due date I can get so I personally would opt for the early u/s to avoid worse (IMO) interventions later (like induction) just in case O occurred later- accounting for the BFN you got just a few days ago. Most woman would test positive by 15 or 16 DPO so I would not be surprised if you did O later than June 1st or 2nd. With this information, you should be able to make a good case for needing an early us/ and you could schedule it for 2-3 weeks from now to be safe- to avoid the need for a repeat u/s if you went in too early.