+1 50gary. There are two main things to consider. First, original lever length. Obviously if you just moved the pegs back a few inches, and also shortened the lever to match, the "leverage" would be so greatly reduced you probably couldn't shift the trans. This of course applies equally to your set up with a linkage. If the lever you borrow from a different bike or fab from scratch is shorter than stock, you have to make up for it by a shorter pedal belcrank lever and a longer shift shaft lever. So in concept, if your lever is half as long, delivering half the leverage, you make up for it by making the ratio of the belcrank and shift shaft levers 1:2. This is actually more complex because the result also is a function of radius vs degrees of rotation, but suffice it to say don't make any of the levers too short, which has a secondary benefit. Because there is always lost motion at any pivot, the longer the lever arms are, the smaller percentage of pedal slop manifested by that lost motion. Imagine a super short belcrank with a 1/16" of lost motion in the rod end or clevis. that would result in a lot of rotation before taking up the slack compared to an extra long belcrank. the second thing is the angle between the various levers and rods. Think of each lever as a crank. When the angle between the crank arm and the rod it is pushing is 90 degrees, you get the most distance traveled for each degree of rotation. It is also the point of least mechanical advantage, so it offers the least power applied. This works backwards when it is the rod pushing the crank, which is now the shift shaft lever, so it balances out for the most part. The trouble arrises when you have a situation when there is a different angle at the ends of the rod. Lets say you have 90 degrees at the belcrank, but the rod is too short to have 90 degrees at the shift shaft. When you lift up with your toe, you are at the weakest point of rotation, but at the shift shaft, the farther you pull the harder it gets because the farther around the shift shaft lever rotates, the worse the mechanical advantage gets. So up shifting will be really bad. Downshifting on the other hand, will be extra good, because as the belcrank rotates forward pushing the rod, it also rotates the shift shaft forward bringing the angle between the shift shaft and rod closer to 90 degrees improving the mechanical advantage.
I have scratch built a LOT of rear sets. To simplify, I suggest tho following.
1) figure out where you want your pegs to be and put them there.
2) now you can sit on the bike and figure out where the pedals need to go. that is the pedal at your toe, not the pivot.
3) If you can make the pedal the same length as stock, it will make things easier, but if you can't, you will have to compensate by belcrank to shift shaft lever ratio. This is the case for your DS, as you have chosen a pedal that is shorter than stock. Your belcrank will need to be shorter than your shift shaft lever the right amount to compensate.
4) pick the pivot location. This is a bit tricky. On the one hand, you rotate your foot like the peg was the center of rotation, but on the other, your foot actually rotates around your ankle, so above and behind the peg is optimal. Fortunately this seems to be much less critical than it would seem. Your stock pivot is the shift shaft, behind and below the stock peg and works fine.
5) Create a connecting rod. Make it the same as the distance between the shift shaft and the pedal pivot. Make it adjustable in both directions, but this will be very close to the best length. The adjustment is NOT to adjust the pedal height, though might serve to tweak it without disturbing the shift quality too much. Don't plan on it though.
6) Now you can determine the angle the belcrank arm needs to be in relation to the pedal lever. I'm sorry to say this is a problem for you because it is not adjustable. Solutions are to cut and re-weld it in the right place, or do likewise to the pedal, or bend it. This is the hazard with using parts intended for a not similar bike. Regardless, the shift shaft lever and the belcrank arm should be parallel to one another, and the angle between each of them and the connecting rod should be as close to 90 degrees as possible. This can not be the case if the two arms are parallel, but split the difference to have them both off the same.
The result will be a good functioning mechanism assuming the pivot axles are parallel, the mechanics are good etc. From there, you can fine tune by adjusting the rod length and spline location for the shift shaft lever, but you should be pretty close to optimal.
Here is a pic of a set up that works very well. Sorry you can't see it at a good angle, but the pedal is short, the belcrank arm is at 90 degrees, the shift shaft arm is longer than the belcrank, and the spline on the shift shaft is one forward of parallel with the rod adjusted longer to reach it. this gives the most power for upshifts, where your foot is weakest, and missed shifts are most common as a result.
Keep up the great work, and don't get discouraged working on the controls. They are deceivingly simple looking, but take a bit of pondering to get to work really well.