There is little advantage tilting -15 degrees unless you need to capture the edge of the panohead footprint.
When shooting approximately -10 degrees gives you the smallest possible nadir hole without capturing the panohead.
That makes perfect sense. I will add this to the database. Unfortunately, not many panoramic heads have -10° click stops or a small enough rotator footprint.
Yes, quite true. If the size of the nadir footprint is important to your workflow you need to give serious consideration when selecting a panohead. Most of Nodal Ninja range is fine giving a smallish footprint. Where as most of the 360P range leaves quite a large footprint.
Basically if the rotator is larger than your tripod mounting flange it is not the one you want when seeking a small nadir footprint. The Nodal Ninja R-D3L panohead with built in levelling base is about as large as you would want and even then you should align the thumb adjusting screws with your tripod legs to minimise infringement.
You can see it is incorrectly positioned in this image.
You can clearly see that if the thumbs adjusters were to be positioned over the tripod legs they would be less intrusive and they would be less to patch/clone or cover.
Also the less tension knobs for ballheads and centre columns in view the better also. Elevating the lower rail from the rotator will also decrease the footprint size. Agnos offer the A-CLN
Adapter Column for just this improvement.
As for click stops on the upper arms, most panoheads don't offer this feature anyway. But yes, the Nodal Ninja 5 has lock stops at 15 degrees and the new M1 series at 7.5 degrees. I think from memory the Agnos MrotatorU is also 15 degrees. Still, with all of these you can manually place the tilt at any position, simply ignore the click or lock stops. I've seen others try many inventions to get the -10 degrees from drilling a hole and placing a pin through, making a groove with a hacksaw and lining up the slots, custom printed plastic moulding and even things as simple as reflective tape that is easy to align by sight even in low light situations.
Or simply read the demarcation marks and line it up!