Matt,
I prefer not to use check valves. I've heard too many horror stories of them failing when they should have worked. Instead I flood test my system.
Determine the max volume your sump can handle w/o overflowing in the event of a power failure. Dont forget your sump will be containing things like skimmers, heaters, pumps, etc...these displace water, so your sump will be containing less water than before. Subtract an inch from the total water height in the sump. This is the highest level you want water in the sump.
Determine how deep you NEED your sump returns to be. If you need an inch of water over the returns to prevent whirlpools that suck air, then you need an inch. If you need two inches, then you need two inches. Now cut your sump returns down to that max needed length. This is now the siphon break. If your tank loses power, the sump returns can only back siphon water until they hit air.
Fomr there, you just calculate how much water that is. Length X Width X Height, Divided by 231 = you water volume. For example, if your returns are 1 inch below the surface of the water, and your tank is a standard 120 gallon size, (48Lx24Wx24H) you will multiply 48X24X1, (length x width X height), = 1152, divided by 231 = 4.987 gallons of water. So your sump will now need to handle an additional 5 gallons of water to prevent flooding in a power outtage.
Repeat this process for your sump....L X W X H, divided by 231 should allow you to factor the volume of your sump, 5 gallons (in this case) will take up.
If your sump is a standard 30 gallon tank (36Lx12Wx16H) if you allow 3 inches for your emergency water level, you will have a reserve space of 5.6 gallons of water. (36 X 12 X 3 = 1296, / 231 = 5.610).
What you do at this point is mark your sump so that you have a high mark for normal water level. If you make a mark at the 4 inch point, (allows safety margin just in case), and you dont let your sump level pass that mark, your sump wont overflow in a power outtage.
To test, shut off the power to the tank briefly. If it looks like it will overflow, turn the power back on and adjust what you need to. I've been doing this for a few years now and had infrequent power outtages over the years and never had a flood from backsiphoning. I've flooded the house other ways, but not like that.
I prefer this method to the standard small siphon break method so i dont have to worry about snails or other crap clogging the siphon break by mistake.
Hope that all made sense,
Nick