Requirements
The first thing Rui needed to do was find the right location under the nineteenth floor. This mattered a lot and would greatly impact his chances of getting away unscathed. The bedrock of the lake was not evenly thick, thus it was probably best that he looked for the place where it was thickest.
Aiming for the thickest part of the bedrock may end up being the hardest and most time-consuming, but it would certainly be the safest. Thus he needed to pay attention to that, among other things.
On top of that, he also needed to pay attention to the tunnels in the vicinity of the point where he was hoping to break through. If the tunnels all lead downwards, then that point was unsuitable. What he needed was a place where there was a route upwards as well.