Hey Eric L,
First, the disclaimers: I shouldn’t even attempt this over the Internet. Please only use this insight as food for thought or for questions to pose to a qualified fitter you work with in person.
Since I can’t see you and measure you everything I say here could be TOTALLY WRONG.
OK, now that we have that understanding:
I’ve done a lot of experimentation with my own measurements and position and fit thousands (literally) of triathletes then been privileged enough to see many of those athletes actually race, getting feedback and photos of how our positions work in the real world. I spent an entire season photographing our customers from the back of a motorcycle in local races. I learned a lot about what does and doesn’t work in the real world with real customers. I’ve also done over 200 triathlons myself so I am familiar with the sport- been in it for over 19 years.
So…
Configure your saddle clamp in the forward orientation on your P3. I think you will need a 110 stem. Chances may be as high as 70% that is correct. I would postulate there is a 15% chance you need a 100 stem. There is about another 15% chance you’ve got some pretty unusual dimensions and I am way off base.
Now, Position the saddle in the center of the rails. Put the bike on the trainer and establish the correct saddle height. Go for 148 degree of leg extension to start. Also, measure your old position from your road bike- very precisely- to see what it is as a benchmark comparison. It may not be right, but it is data worth considering.
Now, Let’s get your angles and dangles right: We need your skeleton to support your upper body, not muscular contractions in your upper body. I need to see a ninety degree angle between your upper torso and bicep, and no more than 100 degrees from your bicep to your forearm. Your elbow pads need to be under your elbows, supporting your weight effortlessly and without relying on grip strength and your latimus dorsi (sp?) muscles. NOT like Armstrong and Ulrich, for what you and I are doing, they are wrong.
Now, the tough part: We have to find the “sweet spot” or the best orientation of your pelvis over the bottom bracket. We have some formulas here for doing it based on Overall length of upper body (torso+arm) but I won’t show those on the Internet.
I think the best saddle orientation for your will be considerably farther forward than you may have envisioned. Remember, we are building a position not only for your bike but also so you can run off the bike. That is key. It will be very different from your road position.
Well, hate to leave you hanging- but without seeing you I frankly don’t know how to proceed from here. Sometimes people send me photos of thier position and there isn;t too much I can do with those either. Positioning needs to be done in a dynamic environment. I need to see a person pedal. Once you’ve watched thousands of triathletes pedal in a trainer you can recognize when they are way off, pretty close and right on. It isn’t rocket science or some mystic BS. It is practice, experience and common sense. I sure wish I could see you in person.