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Oh dear, I must be a real cheapskate.  Fiddling in my workshop with an EBCO I bought on eBay, I used the roofline of houses as far away as I could see.  Then I estimated the vertical distance between the centres of my index and horizon mirrors, took the horizontal distance using Google Maps, and calculated a correction using the one in sixty rule.  E.G.   1 unit up for 60 units along equates to 1 degree correction. So, 1 unit up and 3600 units along equates to 1 minute correction.  Therefore, if the vertical distance between mirrors is 5cm and the horizontal distance to the roofs is 100m the correction is 5x3,600/10,000 = 1.8’.  So, if your real index error is zero, your sextant would be reading minus 1.8,’ and you would have to add that to get a true index error reading.  I.E. If you measured minus 2.8’ your actual index error is minus 1’, which is OK for fun astro with an EBCO, but probably not good enough for serious work.  DaveP