I’ve never made any from pre-pickled options. I have made home refrigerator hot and sweet pickled baby carrots, and… well, I’m abby-normal, they weren’t hot enough for me.
And lots of pickled jalapenos which, well, not hot enough for me and lots of work.
More on topic, but IMHO, both the pickling and the sweet elements tend to mute the heat. Especially the sweet part. I’d absolutely stick to the low end of that sugar range, though it depends greatly if it’s a 16 oz or 32 oz starting jar.
Especially to my heat-warped perceptions, 1 teaspoon of store store bought red chile flakes per POUND of pickles is going to be imperceptible. A teaspoon isn’t a lot over that much volume, especially with “generic” red pepper flakes which loose their potency fast. Same for one fresh jalapeno (which can range from potent to zero heat).
For a mild but noticeable heat, I’d instead want say one fresh serrano pepper, ideally when more ripe so yellow-orange-red coloration being perceptible per pound of pickles.
The problem there (and the one I think your recipe is trying to address) is that it has the gherkins sliced into discs. So any fresh chile option sliced as listed means you might end up getting a hotter-than-expected bite, which is why I advised the color distinction.
Alternately, one ripe serrano, sliced and deseeded (leave the ribs for heat) per 16 oz jar. The slicing will allow the capsicum to spread out more easily, but it’ll be much easier to avoid getting too hot a bite because you can easily identify it.
Good luck!