I read your article in the newsletter. I think you probably have better arms for the seat positions as compared to the distance to the back of the seat. However, I may or may not agree with your CG range of 28.4" to 36". I used the bottom of the firewall as datum. It's five inches ahead of the top. If you have used the top, then we are very close with our CG range. However, your empty CG of 37" puts you aft of your CG range. Adding a pilot with an arm of 65" just doesn't work.
A little explanation about my sketch and the numbers for the Sta 34.00 (18% MAC) and Sta 42.50 (35% MAC). Those are the arms for the CG range using the bottom of the firewall as datum. The CG range for the Clark Y airfoil is from 18% to 35% of the Mean Aerodynamic Chord (MAC) as best I can determine. Since the Hatz is a biplane we need to take the average chord of the two wings. That is the heavy outline I have drawn between the wings on my sketch. My MAC is slightly different than what Randy Brooks (NX24B) shown on his sketch available from the Hatz web site. Because of the center section, I assume the top wing has 54.5% of the lift and the bottom wing has 45.5% of the lift. It makes 0.15" difference in the location of Randy Brooks' CG so not a big deal.
Having said all that, tandem seating airplanes are very forgiving CG wise. I just used a theoretical weight of 500lbs on each of the front wheels and 50 lbs on the rear wheel for a total of 1050 lbs. Plug that into the formula on my sketch and I get on empty CG of 30.9" back from the firewall. This is different than the 37" shown on the HC plans and 38" shown on the CB-1 plans. Neither of which I believe. However, with an empty CG of 30.9" the plane is nose heavy empty. The empty CG is ahead of the forward CG limit of 34". But put a 106 lb pilot in the pilot seat and the CG moves back to 34.0" which is the most forward CG to safely fly the plane. Add me as the pilot and the CG moves back to the middle of the CG range. If you look at your arms for the fuel and the passenger, both sit within the CG range or close to it. So adding either will move the loaded CG very little. I used full fuel, two 200 lbs people and 50 lbs of baggage and I'm still within the weight limits and middle of the CG range.
As far as the trim, I thought about that a lot also. Definitely the position of the engine will affect the CG. Not the CG range, but the arm of the empty CG and all the loading. If the mounted engine lifts the tail of the empty plane off the ground I will be in trouble. I wont be able to get out of the plane without it kneeling like a Long-Ez
The location of the loaded CG will affect how much the tail will have to push down to counterbalance the lift of the wings. Given all the different variables of the different builds and engine configurations there is probably no one incidence of the horizontal stabilizer that will satisfy all of the variations. I believe that is why many of the recent HC builders have made the horizontal stabilizer ground adjustable. It's probably a good way to deal with it.
Personally I checked with flying HC's with the Rotec R3600 engine and build my engine mount similar to theirs. The engine CG is 1.8" farther back than the 0-320 mount shown on the HC plans to make up for the heavier engine. My horizontal stabilizer is ground adjustable and I haven't decided where the battery will go. I'll probably do a preliminary W&B prior to installing the battery so that it's in the best position for weight and balance purposes. I will adjust the incidence of my horizontal stabilizer during the 25 hour test flight time.
I don't know if this answers any questions you have, but because your empty CG of 37" is aft of your CG range of 28.4" to 36" you might want to recheck your calculations. Most of the loading have arms greater than 37" which will move the CG even farther back.