jIH:
> jIHaDchoHta'DI'
SuStel:
> jIHaDchoHDI'

I was trying to express "as soon as I had started studying", trying to give emphasis to the fact that "my starting" had been completed. But now I wonder: In the {jIHaDchoHta'DI'}, is it possible for the {-ta'} to refer to the {-choH}, or does it exclusively refer to the verb ?

Saying jIHaDchoHDI' doesn't mean the starting of your studying is incomplete. It does mean that studying has started. Although it isn't described this way in TKD, -choH is a kind of aspect just like -pu', -taH, and the other type 7s are. Aspect describes how an action behaves over time. -choH describes the action as suddenly starting, then continuing from that point. Combining -choH and -DI' is enough to say "as soon as this starting point arrives."

Whether -ta' must refer to only the verb or can be applied to the -choH is not a settled issue, but I don't think you need to resolve it to say what you're trying to say. You're interested in a starting point, not a stopping point. -ta' describes a stopping point; -choH describes a starting point.

Or to look at it another way: at what point do you finish starting? Can you point to the moment that -DI' points to, when the starting is completed and something else happens?

-- 
SuStel
http://trimboli.name