I'd recommend pricing on a CPM rather than a flat fee. That way you're not leaving money on the table if it goes over and if it's under, you just charge on actual delivery. For example right now you're charging a $20 CPM (cost per thousand) based on reach. But if you deliver 6k, then your rate effectively drops to $16.66. If you say you'recharging a $20 CPM and you estimate getting clients 5k-10k impressions, you're guaranteed to be paid for all the reach you delivered.

That's the fair way to do it for both parties. Honestly though a lot of brands don't consider reach and only look at how many followers you have. You could probably get away with charging some brands a CPM based on your followers which at $10 rate would be $400 a post or $20 would be $800 for you :). I think you could get away with that, but you'd probably need to increase your content to ad ratio to ensure strong performance for your partners