Efficiency calculation help

Let me make sure I'm understanding this correctly....

Is it true to say you can calculate your OG by looking at your pre-boil volume and gravity reading? Here's an example:

I have a pre-boil volume of 6.6 gallons at 1.040. If I plan to boil down to 5 gallons, can I expect an OG of 1.052? I'm I thinking this through correctly by assuming G1*V1=G2*V2

Thanks

 

Yup.

 

That's correct sir.

 

Okay so allow me to take this a step further. Let's say I have a recipe that has a target OG of 1.055 assuming 80% efficiency. I'm getting 70% efficiency. If my pre-boil volume was 7 gallons of 1.033 wort, then I would need to boil my wort down to 4.2 gallons to achieve my target OG. The only other option would be to up my grain bill if I didn't want to boil it down that much OR increase my efficiency by 10% either through better crush, mash, etc...

am i on track?

 

Sounds good to me.

 

that's exactly right