"Optimum" depends on the frequency or frequencies of operation and on
the antenna system. What you need to do is go to the Micrometals web
site (Micrometals is probably the manufacturer of the core) and find the
inductance per turn squared of the core. Then you can calculate the
number of turns needed to get the required reactance at the operating
frequency. For nominally matched antennas, an impedance of 500 - 1000
ohms is generally adequate.

You might find you'll have trouble getting enough impedance at the lower
HF bands. Powdered iron is usually a poor choice for a balun core
because of its low impedance per turn squared. You might pass the self
resonant frequency by the time you get as much reactance as you want.
Ferrites such as type 43 are usually a much better choice -- you get
much more impedance, it stays relatively constant over a very wide
frequency range, and you'll be free of resonance effects.

Roy Lewallen, W7EL

Fred W4JLE wrote:

Using a T200A-2 for a 1:1 balun, what is the optimum number of turns and
best wire size? 200 watts max power.