Although I think FB would be a better prestige move, IS is fine. That being said, 1403 is a decent CP for prestiging. Just think, 2x 1403 = 2806. A lot of dragonites are 2800+, so it is in the vicinity of the exact CP you really need.
Most prestigers' CP I use are between 1000 and 1500 since the meta is generally 2500-3000.