Airship Guide

[TechnoTechie]

And as we already confirmed the grade of a voyage is based on retrieval levels.

Something like this if I remember it correctly:
retrieval levels -> bonus exp -> rating. Basically, the most bonus exp the higher your overall rating. And bonus exp is earned from retrieval levels.

As far as the grade (were you refering to the individual sector rating or the letter grade?) being affected by the bonus exp, it seems kind of independent from the exp gain if the latest voyage from one of our ships is any indication. Of the three sectors visited, there were 2 abysmal ratings and 1 satisfactory, but it still resulted in an S-rating (perhaps sector difficulty plays a factor?): http://imgur.com/utyWpvY (blocking the name because it was named as an inside joke and may be considered inappropriate to some)
[parts reference: Bronco / Invincible / Invincible / Enterprise]

And yet another airship went on a one-sector trip, recieved a satisfactory rating, and got an A-rank: http://imgur.com/Zvj5OqB (it also discovered sector 21 without it being displayed in the log ...! guess they still haven't fixed that)
[parts reference: Bronco / Invincible / Tatanora / Tatanora]

Meanwhile, our 4th ship recieved 1 satisfactory and 1 abysmal, and recieved a C rating:
http://imgur.com/jUXqMmL
[parts reference: Bronco / Invincible / Bronco / Invincible]

I can say for sure that the percentage of satisfactory ratings plays no part in your overall rating, and may have something more along the lines of a weighted rating based on the difficulty of the sectors with respect to your either your ship's rank or stats