Hello,
Stumbled upon this idea and wanted to share it since its so handy. It always helps to know where you need to mount your bearing blocks so that when you add the gears everything is the correct distance apart. Here are a couple simple ways to do it in your head. Hopefully this tip will save you some time when you build your next VEX robot.
// Brute-force method for people who like math: //
To determine the number of holes between any two gear shafts, use this formula:
Divide the number of teeth by 24 for both gears, add them together, and then subtract one. Like this:
Gear Teeth-----> __ 12 __ 36 __ 60 __ 84
Divide by 24 = → __0.5 __ 1.5 _ 2.5 _ 3.5
For example, the number of holes between an 84 and a 60 tooth gear is (3.5 + 2.5) -1 = 5 holes.
To get the number of holes required including the gear shafts, just add 2 to the number above (one hole for each shaft). The above example will use up 7 ( 5 + 2 ) holes in your metal frame.
As you can see, the ‘Divide by 24’ numbers are easy to remember. They start at one-half and increment by one.
// Simplier way for people who prefer less math: //
An even easier way (now that you’ve seen the formula) is to just assign the gear a “short cut number” starting with 0 and ending with 3. Like this:
Gear Teeth-----> __ 12 __ 36 __ 60 __ 84
Short cut number___ 0 __ 1 ___ 2 ____ 3
Using this method, a 12 and 60 is simply 0 + 2 = 2. A 36 to 60 would be 1 + 2 = 3; 36 to 36 would be 1 + 1 = 2. This method tells you (directly) the number of holes in the metal frames between the shafts for any gear combination.
Works every time! So the next time you have to move a bearing block because you missed, I’m sure you’ll remember reading this. 
Regards,
KHall