A FEW NOTES ON ACHIEVING THE PROPER TEMPERATURE:
Don't ever increase the heat, leave it at medium to medium-high for the entire time.
The first time I tried this recipe, I tried to let my mixture reach 300F on the candy thermometer, but it never did. After about 10 minutes of being stuck on 250F, I could smell the sugar burning and the colour of the liquid quickly turned amber in colour. At this point, there was nothing to do but to throw it out (burnt sugar is so gross, do not try to salvage it).
The second attempt, I let it boil for only a few minutes and it didn't quite reach 250F, when I added the baking soda, it foamed but stayed in total liquid form (also unsalvageable). I just did not cook it long enough - all the water had not yet evaporated and it hadn't reached hard-crack stage.
On my third attempt, it still did not reach 300F. It stayed just above 250F for several minutes, and I watched it extremely closely. The bubbles became smaller and the liquid looked much thicker in appearance. I hung around the pot for a few more minutes keenly sniffing for the smell of burnt sugar and watching to make sure the liquid remained clear and did not amber. Using my best judgement, I removed it from the heat and added in the baking soda. This time it worked!
So if you're mixture does not quite reach 300F don't stress about it, just keep a very close eye on it!