For a pure DC voltage, the voltage would be the same anywhere on the coil. The same is not true for RF on the coil. Hi Q coils can develop some very high RF voltages.

I've never seen any direct info on your question. Spacing of the turns is largely determined by the amount of inductance required.

Hi-power circuits will generally use large coils. Larger diameter coils have a greater amount of inductance per turn. Based on that, the turns can be spaced a bit more for a given amount of inductance.

To try to answer your question, look at the voltage ratings for tuning capacitors. 1/4" spacing is good for about 9KV peak. A coil with 9KV across it would not need 1/4" spacing. The coil has a certain amount of inductive reactance. This reactance is distributed equally around the coil. So, the voltage drop around the coil would be same for each turn.

So, if you had a 10 turn coil with 10KV across it, you would only have about 1KV between any two adjacent turns. So, for 1KV the spacing can be close, off hand I forgot exactly what it is. I think an ordinary BC tuning cap spacing is good for about 1KV.

So, in summation, if you have a coil with 1/4" spacing and you're getting flash over between turns, you need to shut off all your equipment and take a break.

Fred