do i have to train early on in the game?

[Renmiri]

I worked out that there were 13 summons so I thought, if I divide 100 by 13 and make sure I am at least that many levels higher than before when I get the next summon, then I'm the right level. Make sense?

Not really because like X, there are 8 summons that are not in the game story line.

I've got 2 "rules" that seemed to work:
1)Match or surpass the level of your Guest
Worked well until Vossler (obviously), after him the only guest you get is late in the game - Reddas - and he comes always 2 levels above your party
2)Level 5-6 levels before each new dungeon
I didn't do this one "religiously". I was having fun hunting for gil and rare game or doing hunts so it worked out that I was 5-6 levels higher every time I had a new monster infested area on the game story line.

You will get the hang of what works for you around the time you get to the southern continent.