As mentioned, the JVN design calculator is an excellent way to pick your ratios. To makes things easier, we have also incorporated the math that both JVN and Ether have discussed here on chief into a web-based
drivetrain calc. This already has all the VEXpro/WCP gearboxes pre-loaded into it, you don't have to go searching for the exact teeth numbers and copy them elsewhere.
The calculator outputs a simple speed, the output speed * one simple efficiency number, plus a loaded speed, calculated by running an acceleration model over a certain distance, factoring in multiple drive train and electrical losses.
When picking your ratios, you'll want to take into account your robot and your drivers. Faster high gear speeds (~18FPS) allow you cross the field quickly, but are more difficult to control. Slower speeds (~14FPS) give you more control, but its going to take longer to get places. You can play around the "Test Distance" variable on our calc to see how different speeds affect how long it takes to get places. Low gear ratios depend mainly on whether you favor pushing, or speed. Slower speeds are great for pushing other robots (less current draw), high speeds once again favor quick movement.
Let us know if you have any more questions