Use a simple calculator, or you own brain if you're a smart arse at maths -
Amount of juice you want to make X target nicotine strength divide by base nicotine strength will give you the amount of nicotine base that you need to add to get the required strength.
You can use whatever nicotine base strength that you like and the calculation always remains the same.
Examples
1. You want 10ml of juice at 3mg nic strength using 72mg nicotine base - 10 X 3 divide 72 = 0.41ml
2. You want 10ml of juice at 3mg nic strength using 54mg nicotine base - 10 X 3 divide 54 = 0.55ml
3. If you really do mean that you want 0.3mg juice rather than 3mg then, for 10ml, using 72mg nicotine base, it's 10 X 0.3 divide 72 = 0.041ml ( and you'll need a very very accurate syringe!!
)
Extremely silly examples 
1. You want 15.5ml of juice at 5.8mg nic strength using 64.5mg nicotine base = 15.5 X 5.8 divide 64.5 = 1.39ml
2. You want 107.3ml of juice at 13.2mg nic strength using 33.3mg nicotine base = 107.3 X 13.2 divide 33.3 = 42.53ml
ps. I only do calculations at night because I'm a Mothematician
