I make my DIY eliquids using 100mg/mL Nic Base in 100% VG also. Mine are all made to be a finished Nic of 6mg, no matter what size batch I make. When I make a 100mL batch that I want to be 6mg Nic when finished...I use 6mL of the 100mg/mL Nic Base in VG to add to my plain PG, plain VG, and various flavorings in the recipe.
So, if your calculator was used correctly...you should have used just 1.8mL of your 100mg/mL Nic Base in VG (along with PG, VG, and flavorings) to make your existing 100mL bottle of eliquid at a finished 1.8mg Nic.
The difference between that 1.8mL Nic Base you should already have in your 100mL bottle you made...and the 6mL total you need to have in it to be a finished 6mg Nic...is 4.2mL more of your Nic base. That amount needs to be added to your existing 100mL bottle you made.
Again, that's ONLY if you used no more than 1.8mL of your Nic Base in the 100mL bottle you made already.