Using a ballistic calculator in this way is my favorite way of doing it. When I first started shooting long range I thought a chrono was mandatory, it's certainly not.
As stated above, testing and verifying those numbers is a must, and atmosphere can play a big role at longer ranges. When testing I always write down atmospheric conditions right before I head to the range and right when I get back, if you have a fancy gadget that reads wind, temperature, pressure and humidity at the range thats even better. At 300 yards it's probably not a big factor but spin drift will make a difference also.
I started using 140gr VLDs for the first time last year, I zeroed at 100 then got drops at 300, 400 and 650 yards. That got me very close at 950 and only 6-8 inches high at 1200.... to be fair to the calculations though, I may have been shooting a hair under 1200.