diy e juice calculator

can anyone help me to Modify existing eliquid from 70 pg / 30 vg 6 mg nic with the aim of reaching 70 VG / 30 PG and 3 MG nic. the thing is this liquid with 70 PG and the 6 MG nic is harsh for me

As title says I just got my liquid barn setup and would much rather mix by weight seeing as I have a 2 digit scale (.01). My computer isn't hooked up right now so was curious if anyone has used a calculator app in the past for weight and which one? Thanks!