Those of you who use boxes.py I have some questions…
For thickness of material, the default is 3, do I leave that alone or…?
The burn is default 0.1 and it says set a higher number for a tighter fit. Will this “likely” (I know nothing is perfect) be tight enough that I won’t have to glue the box together?
This needs to be the actual thickness of the material you want to use for your project.
So for thickness, of say the proofgrade stuff which is 1/8 do I write 1.8?
Measure your material. It’s not always the exact same thickness. It can be off by .1-.2mm sometimes, which is enough to cause joints to be too tight or too loose when measurements are wrong.
1/8 " is expressed in decimals, so it would be .125". All of the thicknesses are in decimals. As jbpa said, always measure your materials. There’s a bit more leeway with that when you’re using Proofgrade, but even then you can’t assume.
First image is what happens when I change it. Second image is what it was…
The measurements are in mm (see where it says “100 mm?”) - so you just changed the material thickness to 1/8 mm, which is very, very tiny. 
I am super confused… so what should I be changing it to? Oh wait… 1/8th inch is like 3.175 mm so I need to change it to that then?
Measure it first, because with wood, 1/8" isn’t always exactly 1/8". Sometimes it’s actually 3mm.
Yes. 
Boxes.py uses metric measurements only. A set of calipers that reads both metric and imperial are your best friend when using boxes.py. 
I’ve just started doing everything on the forge in metric… but I still cant drive 55!
Where it says 3, you must insert the real thickness of what you are cutting, in Millimeters.
Where it says Burn, if you increase that number (it’s in Millimeter too) you will get a tighter fit. But don’t increase it much or the pieces won’t snap together (I use .13 or .15 sometimes)
Oh! Okay, the burn makes a lot of sense. As I adjust it I see the lines get thicker or thinner and that makes sense. So say if I wanted the top to fit on looser, I would make the lines in that area/those connectors thinner?
Next question, why does it have more/less or bigger/smaller tabs based on material thickness? Does it change that for a specific reason like structural stability?
I don’t fine measure my items. Most of my materials are 1/8inch, which is in the ballpark of 3mm so I never change that. I leave the burn alone as well. The fit is tight and seems like it won’t go together, but it does. This is for wood/MDF.
Good luck!
The tab size allows you to adjust the size of your tabs, usually for looks.
If you notice the tabs that hold this frame together are part of the design.
Love this frame! A really fine job on it.
Unfortunately, it is not my design. I copied it from someone on the forum but can’t remember who made it first. I made this one only for myself. I wouldn’t sell any one’s design. 
just to make sure things are clear, here, you won’t get that pattern straight out of the box making program. you’ll get a regular pattern of those small fingers and you’ll need to edit the SVG in a vector editing program to remove the ones you don’t want and make those lines straight on both edges of the box.
but what david did is a very nice job of taking the finger joints to the next level of design.
one other thing to mention with the “burn” setting. as you go forward, you may want to test some simple joints in new materials. every material will have a different kerf (the amount of material burned away). i’ve found that the default there (.1mm) works really well for me for .125" baltic birch. it requires just a little bit of work with a rubber mallet to get the sides together for a perfect friction fit, no glue required.
