In my last Curious Cook column for 2010, I write about the advantages of cooking down candy mixes in the microwave oven, which is more forgiving in several ways than the stovetop. I also give recipes for nut brittle, New-Orleans-style pralines, and saffron-flavored Turkish delight.
For any home cook who enjoys candy-making or would like to give it a try, I strongly recommend Peter Greweling's recent book Chocolates and Confections, in the Culinary Institute of America's "At Home" series (John Wiley & Sons, 2010). His Chocolates and Confections: Formula, Theory, and Technique for the Artisan Confectioner (Wiley, 2007) delves deeper into the technical details.
And for an eye-opening review of just how complex the chemistry of candy syrups is, take a look at the review cited below.
Candy recipes often specify syrup boiling temperatures down to the Fahrenheit degree (half a Celsius degree), because the boiling point is an indicator of sugar concentration and a predictor of the final candy texture. But the review notes that even in carefully controlled lab studies, to say nothing of recipes, "the data available in the literature can vary quite widely and are notoriously inaccurate at higher temperatures."
An example given for a simple 94% solution of table sugar: one lab study reports the corresponding boiling point as 261.5°F, but another as 276.1°. Add corn syrup and other ingredients to the system and there are even more sources of variability.
So don't worry too much about achieving precise temperatures. Cool syrup samples to make sure how they're going to set. And it's not necessarily your doing if your candies vary from batch to batch!
R.W. Hartel et al., Phase/state transitions of confectionery sweeteners: thermodynamic and kinetic aspects. Comprehensive Reviews in Food Science and Food Safety 2011, 10:17-32.