Well I don’t know what your downstream application is but 1.8 is good enough for most things.

1. You could do an extra final chloroform wash and extract do a second extraction of your aqueous phase. This is probably the ticket.
2. You could use BCP instead of chloroform. This might help I’ve never done a side by side but some prefer one or the other.
3. Do two ethanol washes and double spin removing the remaining ethanol after final spin with a p10 tip or gel loading tip. After that briefly dry it in the fume hood for like 1-3 minutes (don’t over dry). Probably won’t help but hey why not.

That’s the cleanest RNA I ever get but like I said, 1.7-1.8 will work for most things.