I normally use one or both of our 8 quart pots for cooking down the raw processed tomatoes. Also, I use anything that's on the counter at the time I start things. My formula (so far) has been that the two pots, as full as I can get them and still let a simmering boil start, will yield 6 quarts of "juice" when I'm done. This is what I put into the jars for canning and can then make sauce or salsa from the concentrated "juice". The only thing put into the "juice" at canning time is some canning salt and possibly a conservative bit of sugar to achieve the sweetness I prefer.
I don't know what the weight of the tomatoes are when I start. I use some large popcorn bowls for the cut and chopped up tomatoes which have the seeds removed if I'm saving seeds from a particular variety. Otherwise, the seeds will be removed by the "Back to Basics" food mill. Six of my popcorn bowls piled high with tomato chunks will yield the two 8 quart pots full of squeezings to reduce down.
Let's see ..... lots of toms ... six makes 6 .... or was it ... 8 is filled by .... anyhow, this is like chinese algebra .... tweak the formula at the end and ....
Well, folks, in any case, it works for me. LOL