Made a quad. Overshot gravity. Low on volume...so..halp?

Depending on the heat source, possibly. If the heat source gets a good vigorous boil for 3 gallons but barely moves 5-6 gallons then no.

 

Yea I would use my brewing burner. I suppose I would have to try and get the same vigor to the boil as I would 6 gallons of wort.

 

Does it come with a sight glass? Does the sight glass have volume markings on it?

 

As already stated by others, keep the equipment/process (including boil vigor the same) and the boil-off rate in volume per time should be constant.

 

This is how you measure kettle volume without markings on the kettle or a sight glass:

Get yourself a food safe metal rod and an accurate way to measure a gallon or liter. Pour precise amounts of water into the kettle. Insert metal rod. Mark the metal rod in relevant increments.

Measure pre-boil volume and post boil wort volume with your handy new wort volume measuring stick. Determine your boil off rate. Update software boil off parameter.

 

That's what I was going to suggest as well. I have seen some people use wooden sticks, but if you take the measurement with it pre-boil on wort, then you are transferring bacteria into the kettle post-chill for the final post-boil wort volume.

 

I use a 1 gallon plastic measuring device (it it what I send my first vorlauf into, how I transfer sparge water, use for measuring, and mix my sugars with hot wort before adding to the kettle, etc.). I fill it with 1/2 gallon of water, dump in kettle, let it settle down, then mark it on the sight glass. Repeat for entire kettle. I also did this with my fermenters. Really helps to dial in the system and figure out efficiency. How much wort did I collect before boiling? How much was left after boiling? How much made it from the kettle into the fermenter(s)? I just need to find a way to do this for kegs as well so I can know exactly how much went into the keg.