Mashable tested the numbers by evolving a CP 630 Drowzee, which the Toolkit calculator predicted would reach between 1310 and 1316 CP as a Hypno. What they got was a Hypno with CP 1306, which isn't in the predicted range, but pretty damn close. In their original Reddit post, Kyurun also warns that the predictions might not be 100% accurate, but they should give you a good indication of what you need to build a killer lineup of Pokémon.
If you REALLY want more accurate numbers, you'll have to spend some time plugging in Pokémon's stats into an IV -- or "individual values" -- calculator, like the one created by Poké Assistant, according to the report. If you have no idea what the hell an IV is, don't worry -- it's not information that's displayed to you in the game. This Reddit post from Conan-The-Librarian offers a simple, ELI5 breakdown, if you're interested.