Since an f4t5's are voltage is 29vac, would an f8t5 be 58vac?

Or would it be volts per like inch between filaments? in that case about 5vac per in.

Or can it be multiplied from the wattage? In this case w*7.25

What would be the arc voltage of an f10t8 and f10t12?

Arc volt is measured from two points at the lamp such as both ends of flourescent  tube while running.   Problem is domt expect starter to work on second lamp in series. Unless the starter is across both lamps at once.  As it takes about 85 volt to trigger the starter. The most I ever got lit is 4 tubes in series on manual preheat on 128 volt
 Line voltage.

How would you measure it from the two ends?

On preheat u can measure it by removing starter while lit.  And I use a modifeid starter with my meter probe leads soldered to starter pin.
This allowed me do quick safe arc voltage check of fluorescent lamp
 In a preheat fixture.

The right test equipment helps, I use this, which can show both voltage and current on the same display. You just put the fork around one of the wires to the lamp and connect the black probe to ground.

Yeah it's expensive, but I'm a test gear junkie, that's my excuse and I'm sticking to it 

@funkybulb Then test ac voltage on the two empty pins?
@Rommie Can I just connect the red one to the ballast output and common to ground?

You could, but a safer way is to just connect the black lead to ground and then put the live (hot) wire to the lamp in the open fork. That will give you lamp current and voltage on a single display and you don't have to make contact with any live wires.

It's an expensive piece of equipment, but it's useful for a lot of things. I wouldn't justify buying it just for this 

I'm guessing hot is red. I should then use amps and voltage to find watts?

Almost, as it's an AC inductive circuit then what you actually get is VA (volt-amps) which is apparent power and is always a higher figure. You need to multiply this by the power factor to get actual watts.

Using this actual tester, the red (hot) probe would not be connected.

Oh, okay.