I am going to agree with phil on this one. An 8.5-25x50 is one big azz scope. I find that my 6-24x40mm is large enough for my 308. This rifle is going to be my 300+ yard ground hog buster ( I hope).
I would go with a 6-18x or 6-24x scope in the end. The lower end of 6x provides a wide field of view then if you want to go after smaller varmints you can crank that puppy up to 24x. Your choice in Rifle is good. The tikka is a good rifle. Are you sure you ant to start out with a 308. The weight of teh rifle will soak up some of teh recoil. I myself would say go with a 223 or maybe a 243win. The 243 will allow you to shoot varmints and work for medium size game.