The way that the formula you posted works, it calculates the pb in two parts. Ceil((5)/1e10) [I am assuming that the 5 is the level of the character] will always result in 1. The second part, ceil((5)/4), calculates the remainder for the proficiency bonus. For levels 1-4, this results in a 1, which when added to part 1 gives the correct proficiency bonus of 2. For levels 5-8, this results in a 2, which gives a total proficiency bonus of 3. This continues for levels 9-12 (resulting in 4 total), 13-16 (resulting in 5 total), and 17-20 (resulting in 6 total).