The Viper HS-t is a second focal plane scope are you shooting on 8x or 4x and holding over? if so then the drop is probably correct as the value for each hash mark changes as you power down.
Haven't tried the 178's but I run the 168s in a 22inch 1in10 with LC brass and 42gr of IMR4064, I have a fairly tight chamber and they are going about 2725fps, if you use the velocity comparison formula from AB that put the 178s at 2650ish fps