The calculation ((Level - 1) / 2 + 1) being number of sneak attack D6s is correct and must be natural number (1..10). Why it looks like this is weird (maybe a dev overthought the formula, it happens, maybe float and rounding shenanigans made it necessary) but if you expand it's really ((Level + 1) / 2) which translates to "half your level rounded up".
This formula only works if the result is rounded. Otherwise it does not match 5E rules for number of sneak attack dice. Of course they roll once and multiply by number instead of rolling a number of times.
