Hi Andres, I also use recycled cores when I find them. I usually take the trouble and try to figure out the operating frequency they were working with by observing the type of chip and its timing resistor and capacitor. Observing the number of turns in the primary and trying to make out what flux density was used comparing with Excellent IT. At least I would have an idea more or less what type of material the core has.
As you may have already noticed that different core material take different number of turns per volt. In any case while you are experimenting you can try out what suits best . Keep in mind that ferrite cores tend to get a little hot in operation and do not be afraid of this. The temperature rise is normally calculated at 25 degrees above ambient so reaching 50 and 60 degrees is no big deal. This feels hot to touch but do not be mislead the temperature of the core must not exceed the 100 degree mark. Here is why I always say that adequate tapes must be used (Mylar tape) as other types tend to get hot and soften up or melt at times.
If the core material is unknown then use N27 as core material this will give you more turns for a given voltage. If the turns are more than necessary then the core will loose power, getting it right will get you the best result.
A good calculation you can adopt for half bridge, full bridge and push pull typology is :-
Nr. turns primary =155* 10^8 / 4 X freq in Hz X flux density in Guass (1200-2000) X core area in cm^2
The flux density will vary according to the frequency used. To give you a rough idea at 20Khz you can use 2000 guass for 50-60 Khz you can use 1500-1600 Guass and for 100Khz you can use 1100-1200 guass. You will see that the higher the frequency the lower the flux density used. This is because the higher the operating frequency the hotter the core will get due to losses.
Lastly with an ER44 transformer you will get around 500w and 700w peak at a medium frequency of 60 to 70Khz.
I suggest you read some blog posts that I posted on this website so that you learn more about transformer winding practices. you can also take a look at the video on youtube of me winding a small transformer for smps.
Link
https://www.youtube.com/watch?v=_K3ixhcTYFg&t=1s
Regards, Silvio