spotting scope limits?

I'm using 300yd as my zero, (.308) at the moment and will be moving out to 600yd a 100yd at time, the 20-60x60 spotting scope i have is working fine at 300yds. The questions i have are, 1-at what point will i need more spotting scope, 2-at what point will i need a spotter, 3-what spotting scope are you using? Thanks, blue

With a 60X spotting scope you are probably at your limit (300 yards) for seeing bullet holes. That answers all three questions.

I've been on a lot of 600 yard lines with highpower shooters and I have yet to see a spotting scope that will focus sharp enough to see 30 caliber holes at 600 yards.