mgable, there are many variables in the amount of rennet to use, me, if it's a cut curd type cheese, I always go with the manufacturers dosage recommendations and if too quick a set then on next cut curd type batch, cut back on amount used. Who knows what concentration/type of rennet a recipe calls for.
So my understanding is the bottle says 1/2 teaspoon for 2 presumably US gallons but the website says use 1/4 teaspoon for 2 US gallons? Your first use on the Havarti, gives a clue, I'd cut back from the 1/2 teaspoon in 2 US gallons you used. Also, you could check the manufacturer's website product's info sheet.
On bitterness, from my readings, excess rennet can cause bitterness in cheese not from the flavour of rennet but by creating excessive proteolysis: "Protein in cheese is broken down from casein to low molecular peptides and amino acids by various kinds of proteolytic enzymes". OK, I know, that doesn't help very much.