We figured it by how much money I would need per year to live on plus inflation times How ever many years til youngest is 18. (you can fund the college funds too if you want... We did.... Same with a bit extra to account for the fact that you will likely get a lower paying job after so many years out of the work force and not get as much in retirement so it needs to be accounted for too). You will also want the living spouse to have the ability to pay for child care if need be and health insurance for you and kids (I would look at insurance policies for your family and then times the price of it by four or five to be on the safe side!)
This number will be huge and as such I suggest you get a trust as the beneficiary of the life insurance (the living spouse can be one running it, or a bank, or other relatives... You an choose!) That way if both of you die some person that was less wise with their money will not lookat your kids and think$$$$ and spend it all getting out of debt, or on a brand new house, leave your kids penniless and then ask another reliative to step in and raise them. Same goes if the living spouse get remarried then divorced and has to split the assets 50/50!