I think this is right, but if it's not can someone please correct.
Take the overall nicotine in the bottle, add on the total nic you add then divide by the volume.
e.g.
Your 10ml bottles of 24mg contain 240mg of nic in total (10 x 24)
For every 1ml of 72mg base you add, you'll be adding 72mg to the total nic, but also adding to the overall volume.
So if you add 1ml of 72mg base, you end up with 312mg in total, but now you have 11mls total volume, so divide 312 by 11 and you should get the nic strength (312 divided by 11 = 28.36). Adding 1ml of your 72mg base raises the overall nic strength to (roughly) 28.4 mg.
If you add 2 mls of your 72, you'll be adding 144 to your original 240 (144+240 = 388) but your total volume will now be 12mls, so divide 388 by 12 to get the nic strength (388/12 = 32.33).
If you add 3mls of your 72mg base, you'd be adding 216mg of nic to your original 240mg but the overall volume will now be 13mls (216+240 = 456) 456 divided by 13 = 35.07. So by adding 3mls of your 72mg, you'd be bumping up the nic content to 35mg.
To get spot on 36mg (or near as damn it), you'd need to add 3.33mls of your 72mg nic base.
Before you do anything though, consider that by adding to the volume by a third, you'll also be diluting the flavour by a third if using unflavoured nic base. You'll also be messing with the PG/VG ratios of your liquid.
Hope this helps and good luck.
edit to add: Please take care when handling higher nic content liquid, it's advisable to wear gloves and wash off any liquid that gets splashed/spilled on to your skin straight away.
