It's never an exact science with springs. There are so many variables and everyone posting up their drops use different standards. Hell, even Tony himself compares his drops to others' drops referencing fender lip-to-ground measurements. Wheel and tire packages, air in the tires, long body-versus-short body vehicles, ALC or non, weight in the cargo hold (empty or with the third row installed and/or tool boxes or massive sound systems), people measuring while parked on uneven or sloped driveways, stock springs being worn out yielding a lower starting height, people using different front and rear drops (yes- front drop can affect rear drop and vice versa), variances in the drop spring's manufacturing between batches, etc. Hardly anyone measures before and after suspension changes and when they do, it's often rough estimates or there were other variables introduced during the process. Then, there's the fact that two identically-built trucks might have sat differently ever since they rolled off the assembly line due to simple manufacturing tolerances.
You can use others' experiences and results to get a general idea and might even get the exact same results. It's just not guaranteed.
Apparently, Tony had that 6" spring custom made for the kits he put together and it's not available by itself. IIRC, the DJM 4230 (for the Trailblazer) drops a fullsize SUV about 5". It's softer than stock so if you cut a coil or so to get more drop, you'd also increase the spring rate, putting the load capacity and ride quality closer to stock. I think
@Byoung1330 is running these coils. If you have the BT coils that have the first couple of inches compressed and touching, you can cut those off for more drop. They're only acting as spacers so the ride quality won't change.