Tony, the plex (30-30) reticle is the simplest of all ballistic and rangefinding reticles, IMO. I apply the plex reticles for rangefinding and downrange zeroing and windage on any scope i happen to be using. Here's how it works--
Let's take a look at some of the Leupold Duplex reticles as an example. If u've ever noticed many of the Leupold scopes have a rangefinding tool built right into the power ring. The system is designed such that the avg. buck deer is supposed to be close to 16" back to brisket. What u are supposed to do is when a buck gives u a broadside profile than adjust the power ring until he fits between the x-hair and plex post tip, then read off the yardage on the opposite side of the power ring. This what they refer to as their range estimating system (RES). The system does have some feasability, but there is a better system that allows the user to apply his plex reticle with many different size tgts, and u don't have to adjust the power ring to make it work. It'll work with any plex reticle, and really any ballistic reticle as well (Ballisitc Plex, Nikon BDC, whatever u're using). It's also faster to apply.
The system is based on the mil-dot mil-ranging formula, but just adapted for the plex measurement (subtension). Here's the formula--
Tgt. size (inches) X range of reticle subtension measurement (usually 100 yds.) / reticle subtension (inches) / quantity of gap tgt. brackets (tenths of total gap) = range (yds.)
Now this formula looks complicated, but in practice it's quite simple.
Ok let's look again at the Leupold Duplex. If u have the Leupold catalog it gives u the measurements (subtensions) of all the plex reticles in the Tech. section. at both low and high power. Suppose u have the 3.5-10X VX-III for instance. That plex reticle is 1 of the RES systems noted above. The subtension of the Duplex is 5.4 inch per hundred yds. @ 10X plex post tip to plex post tip. Let's see how it works when we plug all the #'s into the "modified mil-ranging formula" above--
16 x 100 / 5.4 / 1.0 (deer brackets right at 1 "plex unit") = 296 yds.
Now let's finish the ranging chart for our 16" deer recognizing that 16x100/5.4 is a constant of 296, and we can enter that into our calculator's memory--
296 / 0.9 (deer brackets 90% of our plex unit) = 330
296/.8=370
296/.7=423
296/.6=493
296/.5 (x-hair)=592
Now, u may notice a couple things here. 1st off the .5 mark is the x-hair which is exactly the system that Leupold uses for their RES system above. Another thing that can be seen is that the difference between guessing 6 tenths of a plex unit and 1/2 a plex unit is a whopping 100yds. This should tell u that as range incresases so does the inaccuracy of the system. It's an angular system of measurement and it's not perfect. Gotta go-- wife's home. If u go out and measure your 30-30 reticle at 100 yds. (or even 50 yds. will work--but try to get as accurate to .1 inch as possible), u can try it yourself with gallon milk jugs set out at distance or something else, whatever. It's a kick to play with , and will blow your mind how well it works on tgts. whose dimensions don't change.