1. You don't measure overhang (in the sense of getting or matching some number). You simply adjust it, by moving the cartridge in the headshell slots until the stylus is in the proper position, based on the design parameters of the tonearm and your chosen alignment scheme.
The tool for doing this is an called an alignment protractor, which must be compatible with the tonearm in question. The MintLP protractor already mentioned is one such among many (highly recommended, but costly). Free protractors are available for download and self-printing at vinylengine.com and elsewhere, though none of them will come close to the accuracy of the Mint.
2. A 9g cartridge isn't particularly heavy. More like average I'd say. At any rate, and regardless of cartridge weight, the RB-300 gives you two choices:
2.1 Set the VTF dial to zero. Move the counterweight until the arm floats freely and level. Now adjust the VTF dial to the downforce you want. This engages the internal spring and applies downforce in what's called "dynamic" mode. Some say this is superior for tracking warps.
2.2 Set the VTF dial to its maximum (~ 3g) and leave it there. Set the counterweight to achieve the downforce you want (you'll need a scale). This method disengages the internal spring and applies downforce in what's called "passive" or "static" mode. Some say this is superior because the spring, if engaged, is a resonance trap that feeds noise into the arm.
Which method is truly better? Try them both and decide for yourself! Whichever one you choose:
a) a good (digital) VTF scale is helpful. The VTF dial on Rega arms is almost as notoriously inaccurate as the anti-skate device.
b) a few O-rings to slip on/off the end stub will make fine tuning of VTF by a few hundredths of a gram MUCH easier, more accurate and repeatable. This will matter more with some cartridges than others.
G'luck!
