NavList:

Marty Lyons, you wrote:
"Do you think using this dip short horizon is a good method to calibrate the instrument and derive the index error?"

Yes, your dip short method should work, if I have understood you correctly.

"Since I am calibrating, should I add or subtract the dip correction?"

Well, it depends on how you're using it. Could you describe what you're seeing through the instrument when you're doing this? Also, could you describe further how this instrument works? What is an "Abney Level" (that's a new name to me). I assume it has a bubble somewhere?? What was this used for traditionally? Also, could you provide a photo of the adjustment points and other elements of that scale? 

Incidentally, assuming it's a bubble level of some sort, and going with your statement that you don't need height of eye normally, then you could approach the index error question as a simple "unknown" from a known location. Take a bunch of sights assuming zero index error and look at the resulting errors. What index error would minimize those residual errors? Of course, that's all the index error is: the zeroing error. Note that you could do this easily with my GPS anti Spoof app or equivalent: https://www.reednavigation.com/GPSantiSpoof/.

Frank Reed