Warren Barret-- There is a great post about shooting at inclination angles at the below link.
The angle cosin indicator does not work at long range and has no chance of getting you the correct answer at 1500 yards.
The guy at snipertools.com claimed in the thread on A.R. Tactical that special forces use his product. I can tell you that I was just at Fort Polk, Louisiana, where the Joint Readiness Training Center is located (JRTC) and not one single sniper involved in the training from 5th Group or Delta had a Angle Cosin Indicator on their rifle or in their gear. I asked every sniper there that we worked with if they had one, or used one. None of the Urban snipers at Shugart Gordon Center used them either. If you want to learn the right way to shoot at angles learn it from a Pro who can teach it and do it. The method S1 uses works at Ultra Long range, and it is the method the guys that do it for a living use.
I know the moderators get aggravated when guys tell the truth, but try both methods past 800 yards, and you will see who is telling the truth. The ACI is a gadget and the method that proposed with it does not work in the world of long range shooting. Anyone who has actually tried it knows it produces the wrong answer.