Other than what was mentioned about VG and PG. You can increase TH by vaping slowly for several seconds and then inhaling. Puffing on an ecig as you did on a cigarette produces weak vapor and no TH. Long, slow, easy draws generate much more vapor, flavor, and TH.
24mg would be classified differently by vendors as high or extra high. 24mg is good for most vapers and lots of people prefer to go down in nic. Whatever satisfies your need without creating dizziness and nausea is the right strength. Those two side effects indicate too much nic.