Hi Chris, 

>I have a 3C90 core spare and will wind a 1 :
>1 transformer for now, just for isolation. How many turns should I use
>for best efficiency? 

It's primarily not a matter of efficieny but of flux density in the core
(->heating ,saturation)
What is the size of the core and your power level?
Generally I would wind as much turns as feasible on the core to keep flux as 
low as possible.
For best efficiency and low stray inductance tight coupling of the primary and 
secondary winding is essential.

73
Clemens
DL4RAJ 

>-----Original Message-----
>From: [email protected] 
>[mailto:[email protected]] On Behalf Of Chris Wilson
>Sent: Saturday, October 31, 2015 1:16 PM
>To: Mike Dennison
>Subject: Re[2]: LF: Zmag of 45.6 Ohms OK, or should I try for better?
>