I suggest learning to do the calculations, and if you want later, use a calculator.
I tried to explain e-juice mixing to a pharmacy assistant and they couldn't get it.
Wonder what they do with meds?
Here's the secret to getting your strength level correct. I could care less about VG/PG percentages or even flavor percentages.
Get the nic right.
Decide on how big a batch you are going to make. That will be your Total Final Volume.
Calculate how many milligrams of nicotine will have to go into that volume to make the strength you want.
For example, the 25 mg/ml nic you have, has 25mg of nicotine in every ml,
The final strength will always be mg of nic added divided by total final volume.
If you are using VG then your juice may come out on the thick side. I would suggest distilled water, about 5% of the VG volume be added to reduce the viscosity to that of normal PG e-juice.
Learn to do the math, then think about using a calculator.