I started a thread of similar interest on Shooting the Breeze very recently. It's in the HFT/FT section.

Most 'standard' stocks ( typical standard wooden stocks ) are generally called 'Sporter' stocks. The butt of the rifle generally slopes further downwards as it gets further back from the pistol grip. So at the rear end of the butt the top of the butt will be lower than line of the action/barrel. Furthermore the end of the butt is usually angled so that the bottom is further forwards than the top ... as you describe.

These 'Sporter' stocks are generally designed for kneeling and standing shots. In the kneeling and standing positions the action/barrel is lined up just below the eye, for open sights, and a little lower when a scope is used. In relation to that level ( all with reference to the ground ) the shoulder will be considerably lower, and the 'Sporter' style stock's butt will sit naturally in the shoulder. An adjustable buttpad can still be useful for standers and kneelers as it can be tweaked to make that butt fit perfectly to each individual's shoulder.

The problem is ... in the prone position the shoulder is now much higher than in the standing and kneeling positions. It is usually actually higher than the action/barrel. So a true target prone rifle will have a buttpad that sits very high, and higher than the action/barrel. True target prone rifles don't have the down sloping wooden butt that your standard sporter stock has. True prone is shooting up on the elbows and using a sling to take the weight of the rifle on the fore arm. Do a Google search for Olympic 50m Prone photos.

So technically if you were designing/altering/making a stock for pure prone shooting then you would have the butt pad very high and above the line of the action/barrel.

The problem with HFT is ... it is a compromise ... and the rules allow people to not shoot in a true prone position ... but to shoot in a HFT style fore arm/hand/butt of rifle rested on the ground. Not every shot can be taken like this, but most are by most people.

The other problem you have as a springer shooter is that the greatest majority that shoot HFT, shoot PCP. So most shooters are using a PCP and on most prone shots they are resting the fore hand and fore arm on the floor and resting the butt on the floor. This actually makes the shoulder even higher than in true prone. So to fit the buttpad in the shoulder ( where it should be in a purer sense of shooting ) the buttpad would have to be way above the action/barrel line.

However, you don't get to take every prone shot in that low HFT 'rested on the ground' style prone position. On some shots you have to get up on your elbows to get over obstacles. So again that would mean a different position of the butt to fit in the shoulder.

You also have to take 6 shots per 30 shot course standing and kneeling ( 3 each ). So again a high buttpad would be well out of place for these shots.

The only part of the rifle that can touch the ground in prone shots is the bottom of the buttpad.

What most shooters do ( and remember most shooters are using a PCP ) is they set their adjustable buttpads quite low, to accommodate standers and kneelers, and on the low HFT style prone shots they rest the bottom of the buttpad on the ground. So in the prone position their buttpad is nowhere near their shoulder. It's sitting under the armpit and resting on the ground.

So for you shooting a recoiling rifle ...

You need to practice at a range and see if you can get good accuracy from shooting your springer in the HFT low position with the butt rested on the ground. Some find that the butt on the ground, especially if they use a padded mat to lie on, doesn't cause POI problems. Some other springers shooters ( and the odd PCP shooter ) just prefer to shoot in the more traditional prone, elbows off the ground and butt in the shoulder, position.

If you set your buttpad low then it will be in the right position for standers and kneelers, but it won't be in the right 'true' position for the majority of shots, which are prone. As stated above most PCP shooters don't worry about having it in the right position for prone because they rest the butt on the ground.

If you set your buttpad so that when you shoot up on the elbows, with butt off the ground, and in your shoulder, then it will be way out for standers and kneelers. You can get around this by raising your shoulder higher on those shots. Not ideal.

Like I said ... Compromise.

Some shooters shoot with the front hand and butt off the floor but they still stay quite low in the prone position. They set the butt slightly higher that the wooden butt of their sporter stock, and the buttpad actually rests at the top of the bicep rather than in the shoulder. When shooting standing or kneeling it isn't that much too high and a slightly raised shoulder in those positions will get some decent contact with the buttpad.

It's HFT. You cannot adjust the stock or buttpad once you have taken that first shot. You have to find what works for you.