If you're worried about getting on paper at 600m without knowing your m.v., don't worry it's not that bad. You can probably get by without a chrono, in fact.
Hodgdon's online loading info indicates, for .223 Rem loaded with Varget, an m.v. of 2784fps for a starting load and an m.v. of 3010 for their indicated max load. Assuming you have any reasonable loading of Varget, you won't know what exactly your m.v. is but let's take a wild guess and assume it's in the middle - 2900fps.
You have a 100m (or 100y) elevation zero. Run your favourite ballistics program (I like JBM online), with the best ballistics data you can get (there is "Litz" drag data for a 69 Sierra, so I'll use that).
JBM tells me that a 69MK(Litz)@2900fps will drop 18.1 MOA from a 100m zero. So come up 18MOA and fire one or more shots at a clean target at 600m.
But what if you're wrong? What if the m.v. is actually 2784fps? Well then the bullets will strike lower. Instead of the 20.0MOA you should have had on the gun for 2784fps m.v. you've only got 18MOA, so your bullets will group 2MOA below your point of aim. This is about a foot low.
Or, what if your m.v. is actually 3010fps? Well the bullets will strike higher. You have 18MOA up from 100 on your scope but you should have come up 16.5 MOA instead - so your group will hit about 1.5 MOA high (which at 600m is nine or ten inches).
If you have a clean 3'x3' (or even better 4'x4') piece of paper up at 600m, you should be able to catch all your shots. And once you see where they are actually landing, you can then back-estimate what the m.v. probably is.
If you want to make a first-round hit on the 6'x6' target board at 900m or 1000m, you need to know your m.v. You can do this with a chrono, or you can do this by firing at 600m, carefully measuring and correcting, and using your 600m results to guide you to putting on the correct elevation for your 900m or 1000m shot.
