Hi 3-Charlie,
Its not clear to me what you are asking regarding "calibrated", so perhaps you could clarify?
Are you asking about how to use the elevation and windage dials to zero a rifle? I am guessing you already know that you can zero your rifle at your chosen range (e.g. 100 yds/m, 200 yds/m) using these dials clicking up and down, left and right to find your zero point of impact? If you are asking about this there are there are many helpful YouTube videos showing you how to do this.
Are you asking about the hash marks, or dots, or thick-thin sections of the reticle cross hairs and how to calculate bullet drop compensation (BDC), or how to range a target with the reticle marks?
If this is your question, you need to test shoot specific ammo at specific distance and record point of impact relative to your aiming point. Then you will develop a data sheet on how to hold over or under a point using your reticle within specific distances, or use the turrets to dial up or down to achieve point of impact and aim matching.
For purposes of ranging a target, you can calculate what the reticle marks are doing (start with maximum magnification) by posting a scale image marked in inches at 100 yds/m, and take notes on what width or height is bracketed by the hash marks/dots at maximum and lower magnifications, and where the bullet hits.
"Point blank range" (for a centerfire cartridge and specific bullet weight in store bought ammo) is the distance within which you can hold the center of your reticle on the target and expect a +/- 3 inch group on your point of aim. With deer hunting cartridges this is typically within 0 to 230-300 yards depending on the many variables. If you reload you control and refine that distance precisely.
Its true that some reticle hash marks or dots are "designed" for a specific bullet drop trajectory at specified distances like 200, 300, 400 yards (zero'd at 100). But as Jerry points out these reticles are seldom accurate because of the many many variables involved, especially in the variance in ammo components (brass, primer, powder and bullet) and how the manufacturer changes these through time. Atmospherics also affects everything. BDC reticles are not reliable for nice round numbers of 100, 200, 300, 400 yards, etc. But you can use these reticle marks for generating data on actual trajectories and point of impact at various distances.
With some time invested in range work taking detailed notes on gridded targets (I like the shoot-and-see targets gridded in 1 inch grids), you can generate data on where your ammo hits relative to the reticle center and the hash marks/dots for calculating hold overs/unders at various distances.
Long story short as folks have mentioned above, you derive your bullet's trajectory, point of aim, point of impact, using the scope's turrets, and hold over/under distances based on your shooting results. Measure everything on the target results and write it down, and then you have it. Change your ammo or scope and you do it all over again. With practice it becomes quick and second nature. In fact it becomes so simple that reloaders constantly tinkering with differing loads using many bullets, powders and cartridges, rilfes and scopes do this dozens to hundreds of times per year, for fun, and we generate thick notebooks of data as we go.
