I thought I'd chime in and probably add to the confusion! The problem looks like this to me:
1: you used product A (7.2 % nic in a VG solution) originally
2: you now try product B (also 7.2 % nic in a VG solution.
3: Product B seems to be different to product A (thicker, harsher etc)
4: this causes you to believe that product B is a "premix" of nicotine and the requisite amount of VG needed for a final recipe. It seems as if product A was "just nicotine" because it was thinner.
5: Because it seems as if product B is "already done", you think perhaps :
A: No extra VG should be added (as suggested by the calculator) cos product B is already mixed correctly, so adding more would be too much. Or, more hazardously:
B: The combined total suggested by the calculator (3.4ml+ 1.1ml in your example) should be taken just from product B and added to the PG and flavour.
That's what it looks like to me at the minute I think! If you've done what point 5B says, and (to use the screenshot example you posted) just taken 4.5ml of "product B" (Darkstar 7.2% nic) and added it to 4ml of PG and 1.5ml of flavour, then you're going to have a fuck awful lot stronger brew than you were aiming for! That would explain the harshness etc. The confusion lies in thinking product B is "ready to go" as it were. In theory, both product A and product B should be exactly the same. The difference in them you you've found makes you think one is almost "pure nicotine" and the other thicker one is "pure nicotine with the right amount of VG already added". But it ain't! It looks like you had a shitty product A to start off with cos it should pretty much be the same consistency etc as product B. So, basically, what I would do, would be treat product B in the same way as I did product A. It is your "nicotine" in the calculator and it still needs to have the amount of VG, PG and flavouring your calculator suggests added to it to make whatever recipe it is you're trying to make. Don't whatever you do add the VG and nic amounts together and dump whole amount from product B into your mix. It'll be a bit rough lol! I may of course be totally wrong about all of this.. It's late and I'm knackered lol! Best of luck with your mixing adventures!