I don't know how to do it right ... but I know how I do it.

Seat position is invariable. My leg is only going to work with power and without injury at one length, and while I can slide the seat forward or back some, there isn't much range fore-and-aft before i start compromising power or comfort.

Once I get my seat set, it will be about the same on every similar bike (obviously a road bike, TT bike, or MTB will not be similar.)

The same vertical rising from the BB that is used to figure reach, is also used to figure seatpost setback and saddle position. Drop a plumb bob (ties a string around the top tube with a weight on the end) to get a sure vertical to the BB center.

Once the saddle position is determined, to can sit on the bike and reach out and down and see how different positions feel. Get into the drops and bend your arms to see if your body is comfortable and you can spin your legs. When yo know where your torso can fit, well, your arms are connected to your torso.

All of rm-rf's ideas are great. Take out all the spacers, flip the stem if it points up---try holding the brake levers for a couple inches further forward reach, to see if it is puts your torso in a comfortable place. All these can help you figure how long and low you can ride while still both breathing and making power.

Of course ... if you happen to know someone about the same size as you who rides a racier posture ......