Taking Bias voltage reading?

[carboncomp]

When I'm taking Bias voltage reading using a battery do I subtract the over voltage?

For example, If I'm aiming for a Collector voltage of 7V on a boost pedal using a battery that's 9.8V do I want a reding on 7v or 7.8V

Dose that make sense? 

[GGBB]

It would depend on the particulars of the circuit in question - I think.  Specs like that are often written in reference to a 9.0V power supply - in theory at least - so in that case what you would want to aim for with a 9.8V supply is (7/9)*9.8V=7.62V.  But there are cases where you want to aim for a specific voltage regardless of the exact supply voltage, so you would want 7.0V even with a 9.8V supply.  However, unless the circuit regulates the input voltage to exactly 9.0V (which most pedals designed for a 9V supply don't), aiming for 7.0V with a 9.8V will mean that you no longer have exactly 7.0V as soon as the 9.8V battery drains a bit, or if you power it from an adapter that is not exactly 9.8V or is unregulated.  If it were critical to hit 7.0V, the circuit itself would have to provide that voltage stabilized through regulation, and then there would be no need for you to adjust anything.  So by telling you to "aim" for 7V its inferred that this is a ballpark.  In many cases (e.g. fuzzes and such), you're best bet is to determine by ear what sounds best.  Otherwise, best bet is to go with the 7/9 of Vin with a fresh battery to roughly cover the range of voltages that a 9V battery will provide over it's life, or if you are always going to be using the same regulated power supply (e.g. pedal board supply), set it at 7V while you are using that same supply.

[guitarpedalparts]

I would agree with the above statement.  In many boost pedals, for example, it may be looking for a voltage reading that is half of the voltage being given to the rest of the pedal.  Therefore, a 9.8V supply would want to show a 4.9V reading.  There may be other instances, such as with a digital chip, where it would require a more specific voltage.  But those cases would usually be controlled more accurately with a voltage regulator and not a trim pot.

You could always (as mentioned) try listening for what sounds best within the estimated range, assuming you know it won't fry the part.