## Doing it right ##
The Tarrasque's Jump check is not particularly great as it hasn't bothered to put skill ranks into it, just its Strength modifier of 17. It is even negatively impacted by its slow speed, for a total of +11. If it rushes though, it gains a bonus of ((150-30)/10)*4 = 48 instead of a penalty, a great improvement. The total effective Jump skill therefore becomes **+65**, which sounds impressive. 

Unfortunately, it's not. It means that with a running start the Tarrasque can leap about (65 + average roll of 10)/4 = under 20 feet. Which would be amazing for a human, but is less than half of it's quadruped standing height of 50 feet. **The mighty Tarrasque hops.**

As for setting the DC for reaching varying heights, you are correct in thinking that no check is required if the height is within the Tarrasque's reach, and only the difference in height needs to be covered if it's not. Going by the 128 feet of high reach provided by the rules, **all you'd have to do to calculate the jump DC is subtract 128 from the desired height and multiply the result by 4.** 

## Having fun ##
Going strictly by the rules in this instance results in a fairly unimpressive Tarrasque that would have no ability to reach enemies 150 feet in the air. The rules for skill use are written with humanoid and human-sized characters in mind, and strain to keep up the further you move from that norm. 

You can try and keep working within their framework. Maybe, your Tarrasque put skill ranks into Jump instead of Search. Maybe it has a huge racial bonus to Jump, or keeps a wizard around as its familiar, to cast *Jump* as needed. None of these will get you that much further off the ground, but maybe they'll be enough.

Alternatively, consider the approach 13th Age (a sidequel to D&D) has taken in its depiction of the ubiquitous Tarrasque:
>  Note that flying is usually no defense against the tarrasque, which is huge and powerful enough to pluck or whack enemies out of the sky with surprising leaps.

That is the entirety of rules it dedicates to the question.