I'm aware of the effect of slight changes in seat angle, but in mounting a new seat I'm not sure how to choose a good starting position so that I can just vary from there a degree or two at a time. I find that it takes more than just a quick run up and down the street to really tell how a saddle is going to feel, so don't want to have to make too many changes or have to try big changes.

Anyway, many places that I've looked -- such as on Youtube -- say that the optimal starting position is to have it level, which at least gives you a repeatable position if you set it using a carpenter's level. This is easy to do with a typical flat-topped seat, but many seats out there are contoured with a kick-up at the back and/or sloped down at the front, etc. So how do your figure out what "level" means on a seat like this? Putting a level on the seat so it rests on the two high points would be easy, but could easily start you off with the nose too high and make it tough to find the sweet spot.

Is there any way to figure out what the intended "level" position of a seat is designed to be? I always thought it would be nice if the seat rails were made to be parallel to the neutral position of the saddle, but I can see that this is not usually the case.

For a new saddle I measure the distance from the crank to where my sit bones should be on the old saddle and from the stem to the same point on the old saddle. Also, I take a good look at how the nose is tilted or not in relation to where the sit bones should be.

Then I try to get the same measurement on the new saddle and just eyeball or maybe with the help of a spirt level get the tilt of the nose there 'bouts. If the saddle profile is different, I just do what looks right to me. Then I'll go through several weeks of riding with the tools to make minor adjustments until I get tired of tweaking.

See where your butt is on the saddle. Make that part level.

I would agree with the posts 2 & 3. I measure from the position of the sit bones, since this is where one actually sits, and ignore the the rearmost part of the saddle.

What the previous posts have said. Find two repeatable spots on your bike, head tube, bottom bracket, are the two I use, to take your measurements from. I make sure to parallel the seat tube with the ruler from the bottom bracket to the seat, to make the measurement consistent and repeatable. Also, though I realize this is more than obvious, find a level ground area to make your seat level measurement. Straighten the front wheel and get the bike as perpendicular to the ground as possible to set your seat level. If the seat is not dead flat, and few saddles are, start with what looks right and using a spirit level on two contact points, set it so the bubble is level. Then you will need to just ride the bike, there is no shortcut at this point. Also, set your front to back, knee over the peddle spindle, as a starting point and just work on the seat angle for now. Don't do two adjustments at the same time, move the seat forward a 1/4 inch and lower the nose. One adjustment at a time so you will know if that worked or made things worse. Hope this helps.
Frank.

Thanks all. I did a complete remount of the seat and did my adjustments more "scientifically" and after just a few tries I think I have it where I want it. I think part of my problem has been that I'm trying to achieve a fit that was right for me a few years ago, but at 70+ the effects of aging seem to be accelerating lately and I'm just not as flexible as I used to be. So even though my seat seems "right" now, I might try a different stem to raise the bars slightly to make it easier on my neck when looking up to look forward.

To make it easier on your neck, straighten your back from shorts top to shoulders, or as much as you can. Stretching helps a lot. Riding Position Discovery

I had the first bike fit of my life a couple weeks ago, to try to get a saddle sore to go away. To start with, the fitter asked me to try to touch my toes. I put my knuckles on the ground. That's what you want. No, the sore hasn't gone away, but I think it has nothing to do with fit, probably a fungus infection. Gotta try everything though.

I recently bought a used 2017 Giant TCR bike, rode it a few times with the factory saddle and decided it wasn't going to work for me.

Purchased a Selle SMP Well and after a few rides have it pretty well dialed in so that it's comfortable for long (3+ hours) periods of time.

Selle SMP says to find level put a straight edge front to back on the saddle and put a level on the straight edge. Adjust to level and there you go.

While this may not be true for all saddles it helped me immensely with regards to getting a little bit of nose down dialed in to alleviate pressure at the nose end of saddle against my bits without rolling me too far forward.

Also now if I move the saddle fore and aft I know where the bubble was and can find the position again.

Hope this helps a little.