Actually there is a bit more to it than that, I can't remember which thread this was originally published in (It was right around the time GV's were 1st introduced). I can't even remember who posted it but I saved it at the time and used it a lot until I got to "Just know" what range to use as a starting point. You will die a lot while you spend time doing the maths for a while but it does get better. The GV Dimensions are my addition (Taken from various books etc).
Anyway here it is good luck.
A long while ago, hitech gave us all a clue by saying that the horizontal lines in the panzer's sight are 5 mils in length. Knowing this and knowing the length or width of your target, you can deduce the range of the target. Let's say a pz is in front of you facing you head on. You look into the main gun's site and zoom in. Measure the width of the panzer using one of the many horizontal lines... is it 1/5 of a line? 3/5? Let's say you judge it to be 2/5 of the line in width, or 2 mils. The width of the panzer in yards is about 3 yds. We want to use yards here because the panzer site is marked in hundreds of yards ( 4 is 400 yds, 25 is 2500 yds ). I use this formula that I extracted from a book entitled "The Ultimate Sniper" by John Plaster, in a section on range estimation:
target dimension x 1000 / target dimension in mils = range
3 yds x 1000 / 2 = 1500 yards
Target dimension will be based on whether you're seeing the side (length) or the front (width) of the panzer/m3/m8 etc. Make sure you use yards. Try this formula and see if it works for you. There are other ways of using the mil, this was just the way I chose to remember. Also see how things are done in the tank sim "Panzer Elite." The German pz's sights in there are historical, and are a bit more complex in design, but easier to range stuff.
When pz's first came out and were used extensively, there was a bunch of folks who could range a target just by looking at it's size in the scope. That is the quickest way, and requires lots of practice. If you ever get hit on what looks like the first shot taken by your target, you'll know why...
panzer IV & Ostwind
length 6.5
Width 3.1
Tiger 1
Length 9.0
Width 4.0
M8 Greyhound
Length 5.5
Width 2.9
M3/M16
Length 7.0
Width 2.4
LVT (all)
Length 8.9
Width 3.6
All Measurements shown in Yards.
TTFN
snafu