Most of the transformers i wound were for 100 watt or less loads, and the chosen core ratings were close to what was needed in most cases. After finding virtually zero copper loses and comparing that with all of the litter-ature then stating / complaining about copper losses, i took a look into the why was i getting so much better performance? Well, it turned out that the charts appeared to be for "original" iron / steel laminations and all of the transformers i was using had grain-oriented laminations; far superior in power handling per cubic inch of metal. So a size that the chart would call "100W" actually could handle 200W or perhaps more before core heating became significant. This also explains why i always found that there was extra space in the window (no....NOT Vista).