Just to throw in something else, if you put the CO2 tank the fridge the gauge will read pretty low after that first keg but it won't be out. They don't read progressive like one might think. Mine is at 500 PSI and has a lot to go. Then when it's out out it suddenly drops to zero.
If yours is...
Can't say how many times I've learned something by Googling a brewing topic using site:homebrewtalk.com to get results from here. Hundreds I'm sure.
Started out a little over a year ago on the stove top with extract. Currently I'm turning out 10 gallon all grain batches in a keggle on the...
42 to 40 isn't that far off but if you want to hit the recipe OG you can always use a refractometer. It makes it easy to take fast samples during the boil. Of course adding a few minutes at the end to get the gravity up might mess with your late hop additions, but you'd have to make that...