It may seem logical for Austria, if they feel like that, to push the border to the Sesia river.
However, that would seem very unwise in terms of general European diplomacy, as Austria was supposed to be fighting to uphold the status quo, not to rock it (and they knew it). While the relvant territory was historically Milanese and only acquired by Savoy through earlier agreements with Austria (in the Spanish and Austrian Wars of Succession; in both of which, the House of Savoy acted opportunistically from the Viennese perspective and the relevant lands were given pretty begrudgingly) by 1849 Austria had no significant economic, diplomatic or strategic incentive to regain the area (much less if it's only part of it, which would make for a less defensible border).
Essentially, it would only mean to have an even more hostile Piedmont while having to rule even more disgruntled Lombards across the new (still less defensible) border without significant gain in either strategic depth or weakening of the enemy.
Diplomatically, acquiring land would be seen very badly by all other powers, which Austria could ill afford to alienate.
In broader strategic terms, Austrian resources in 1849 were stretched as it was, risking more prolonged/resumed resistance on the Italian front because of territorial demands, while they had to deal with Hungary and Prussia was probably not considered worth the time to consider much (I think a brief thought on it was there). Not to mention the possibility that France would then intervene immediately, which would be outright catastrophe.
In hindsight, of course, nothing of that mattered, and Austria would lose Lombardy in ten years anyway, so they could have gone Carthaginian and who knows, it would have turned for the better for Austria. It may be a fun TL.
But the reasoning at the time had strong reasons not to push things with Piedmont that made a lot of sense.