Candy making isn't my strong suit, but I dug around in my piles-o-files from a class I too back in the dark ages and found this information.....

-Begin with the finest ingredients.
I had an instructor, who also had her own candy shop, and she would only use C&H pure cane sugar and avoided sugar made from sugar beets. But personally, I never quite bought into that theory.

-The weather and altitude can affect candy making. Cool weather and a cool room temperature with low humidity is ideal. In other words, don't make candy after a morning of cooking, baking or canning where the ambient temperature is over 70-degrees and the humidity over 50%.

-Check your thermometer for accuracy. Test it in boiling water FOR 10 MINUTES and see if it registers 212-degrees F. Recalibrate if necessary. You may need to adjust things if you are in a higher elevation. Higher altitudes require an adjustment in boiling temperatures. For each increase of 500 ft. above sea level, sugar syrups should be cooked 1-degree lower than indicated in the recipe.

-I checked a bunch of recipes on-line for comparison, and most of them heat toffee to 285-degrees F, not 300.

-If you are looking for consistency, find a recipe that weighs the ingredients on a kitchen scale.

I've included a link that has "12 secrets" for making toffee plus a recipe where the ingredients are measured on a scale.

Better luck with your next batch :-).

-Grainlady

Here is a link that might be useful: How to make world-class gourmet toffee.