First of all, I would like to thank the great work of the user "unua_nomo" who created a good part of the table and reached the conclusion of the initial numbers.
So please go over there on Reddit and thank him if you think this might be useful.
So, I merged a little table that the user unua_nomo created with the rules used for creating monsters from the Unchained book.
How it works:
Choose the function of the monster you want to create or convert and the level. The numbers are approximate with what we have in the bestiary of the playtest, but it is good to always compare with your party.
For example, Brute has the highest damage of all but one of the worst AC and its to-hit is low, probably only hitting in its first attack. A critic can be very dangerous and his life being the largest of all means that it counterbalances his AC.
If you give the monster any Weakness, try to increase it's HP by the double of the weakness value.
If you give resistance, decreases it's HP by 1/4 (not sure yet) of the resistance value.
Saves are estimated, and should be adjusted case by case, the idea is to give you an approximate value for a good save and a worse save.
Abilities can be created or you can use the lists we have from the Unchained book and Playtest Bestiary, which are very good.
The Skills were separated into 3 groups:
Good: Skills where there is some investment to make it good
Master: Skills where there is the maximum investment to be good
Legendary: Skills that are above normality, such as a Deception of a Doppelganger or a Stealth of a Skulk
Anyway, I hope it can be useful.