I know it seems kinda confusing, but in actuality it really isn't. I like to keep the math simple.
With a bottle of 60mg juice, if you take 5ml of that and mix it with 5ml of something else (PG and/or VG and flavoring) you have 10ml of 30mg liquid. So, if you take the whole 60ml bottle of 60mg juice and mix it with 60ml of PG and/or VG you now have 120ml of 30mg liquid. Mix with equal amounts like that and you get twice the total amount and cut the nic level in half. If you were to take that 120ml of 30mg juice and mix it with 120ml of PG and/or VG you'd have 240ml of 15mg liquid.
The math gets a little harder if you want a different nic level. That's why there are so many eliquid calculators online. If you wanted to end up with, say, 60ml of 24mg liquid you would mix 24ml of your 60mg juice with 36ml of PG and/or VG. Just do a search for a ejuice or eliquid recipe calculator and you'll find many. You input the numbers and it tells you how much of what to mix.
I would start with the 60ml or, as I do, 125ml of nic juice, and the same size bottle of PG and one of VG. You're covered on all bases for a lot of eliquid! I just vary the amount of VG I use with no pattern, just what I feel like at the moment.
Hope this helps,
Steve