Sorry i missed this. Using a pot is not like using a beaker. In a beaker you can see what is going on, but with a lid on a pot it's more difficult to judge the solution level or the reaction kinetics. I usually leave the lid off until i'm just about ready to add the final shots or two of nitric. During the first additions things are a little ornery and you need to fly sort of by the seat of your pants. Then you can put the lid on, but i leave it cocked sideways just a little bit with a crack. This allows the lids to slope and condensation that collects on the bottom of the lid will drain to the low side and drip back down into the pot. When you put the lid on flat the steam coming out the lid around the rim will condensate on the outside of the lip around the top of the pot and drip down the side of the pot and get on everything. A lid also serves to trap heat. As a solution grows in volume the ions have to move further and further to reach their target. So what happens is you think you are running out of acid to do the work, but what is really happening is they are still there they just have to travel further. Heat is that angry energy you need to push them around faster and faster. Getting a thousand or two milliliters up to boiling is not a real big problem, but getting 20,000 ml up to boiling is a real problem. As the solution grows it gives more and more surface area for heat loss on the pot surface. Back in the day i had to move from electric up to a gas turkey fryer because i just couldn't pump the heat in faster than i was losing it with electric. This is in a well vented fume hood pulling a lot of cold outside air in and around the pot which serves as a big heat dissipator in essence. Gas gets you there faster!
When you are running gold filled or silver in a 5 gallon pot it's not like with a beaker. With a beaker you are measuring in milliliters and with a large pot you are measuring in 100's of milliliters if not 1,000's. The common rule for a beaker is 2/3. That means if you have a 1,000 ml beaker the most you want to fill it is about 650 ml or just shy of 2/3. The other 1/3 is for expansion. Same with pots! A gallon of nitric will dissolve a little over 2 lbs of copper and base metals. 5 lbs of GF counting water, nitric, and wash downs you can easily do 5 lbs in a 5 gallon pot and be within the 2/3 rule.