Bill I have a simple experiment for you to give you an idea of what first focal is about.
Find a paper plate, scissors and a Sharpie. Draw a ring around the plate about a half inch from the edge then a crosshair and a holdover point (dot or hash mark, your preference) below the crosshair. Cut out the four quadrants so only the rim, crosshair and holdover are left. Next pick a target, I suggest a tv with a DVD image on pause. With Scotch tape tape your reticle on the screen and have a seat on the couch. Now imagine that you are zooming in with your "scope" and walk toward the tv. Neither point of impact has changed (crosshair or holdover point) as you "zoom in". In contrast, to view second focal plane remove the plate and sit back down. Focus your crosshair on your tv target and note where your holdover point is. Now again "zoom in" by walking toward the tv keeping your crosshair fixed on target. At the halfway point stop and check your holdover. You should note a marked change in point of impact for it even though your crosshair is still on target.
Now imagine that at say 200 yards was your original zero for the crosshair and you found the holdover point to be 500 yards. If you had zeroed at say 6x on the range at a six inch target the holdover point would have appeared to be below target. As you zoomed to 24x while holding the crosshair on target the holdover point would "walk" up the target and likely show a point of impact now within that six inch target. If you next centered the target with the holdover point and fired you would find your bullet's point of impact some distance above the target. Therefore: if using a SFP scope you must know the holdover point for each power setting of your scope. One last thing, lest you think the p.o.i. won't change THAT much, on my Rem. 700 chambered for 30-06 I have a 4-16x scope. At 100 yard zero the lowest holdover point is on target at 636 yards at 4x.
That same holdover becomes a 1,093 yard zero at 16x for a change of 457 yards. That's why, if you can afford one, you should buy a FFP scope or consider lower/fixed magnification. For .308 (ballistically similar to my -06) you might consider a Pride Fowler 3-9x which uses FFP, is reasonably priced, has very good glass and is calibrated ballistically for .308 (well, certain bullet weights anyway, though they do list them). It also gives holdover points out to 800 yards based on those ballistics with such accuracy they now license their reticles to Zeiss (I'm sure you know them) who calls it a Rapid Z Reticle (Pride Fowler originally named them Rapid Reticles, also the name of their website). The shortcoming of the Zeiss is that its a SFP scope thus they are paying royalties to use a reticle that no longer gives propper holdover points. Safariland (the holster maker) has also obtained a license to use the reticles and wisely chose FFP. The PF RR800-1 can be had for a street price of around $550 if you can find it, otherwise you're looking at around $800 direct. Take a minute to view their website as I think this scope would suit your purpose, or 80% of it if you're serious about 1000 yard shots and you can look at their chart to determine whether your particular load fits the parameters (it should). Keep in mind also that those holdover points are etched into the glass so the only field adjustments are for windage which is also compensated for and magnification, read NO MORE MATH! We on this side of the pond seem to be sorely lacking in that discipline and the speed difference between calculating and precalculated certainly couldn't hurt.
