I dont have a formula or do I measure in inches. All I can say is that I do layer sprinkling of salt into the initial bed. Then depending on conditions I add some set dirt or put up dry dirt and pour some salt onto this medium then stir the salt in with my hand to mix it throughout. Make a pocket in the bedding with my hand then twist and pinch pack the dirt under the levers and against the outside jaws real fast to stablize the trap fairly well.I shake a sprinkle of salt around the whole outside edge of the bedded trap once again.
Add a couple handfuls of dry sifted peat to the inside of the jaws area and press the peat into the interior of the jaws to reduce sagging.I then put the finish cover dirt into my sifter pour some salt on the dirt top. Then sift the set over with this final cover.
The salt will mix nicely with the dirt as you shake over the set proper and the apron area.Dont just bullseye the the trap with your cover.Allow some overlap to blend nicely.
This winter set conditioning is all done in less then a minute once the trap is staked and set for positioning.I have done this for so long it goes very quickly.
Step two Method:
If you cut your bed just right, just the size of the trap jaws only,dig the bed 5"-6" deep, hammer down the area for your trap levers, you can just sprinkle salt in the initial cut out bed,stake your trap, fill the bed with the needed peat, bed the trap in the peat, cover the bedded trap with peat,compress the peat the best possible and finish with a salt treated dirt finish.If you practice your pattern cutting to get a tight fit,you can actually pound your trap secure by pounding the soil at both ouside jaw positions just enough to pinch it tight.You need to make sure you dont have any bare steel contact to the soil that is not treated or barriered in some manner, as the moisture creep may grab your trap. Rub salt against the jaw contact soil areas or you can spray your trap frame underside, levers and outside jaw edges with a 50/50 mix of glycol and water to stop the frost grab.
I do things instinctively now depending on expected conditions. Even with the best methods things can be cursed by mother nature.