I'd say best way is to do it by trial and error. Some pointers I've found useful when making DP for my own campaigns:
- Rather too weak than too powerful: Too powerful ones will make the game boring for EVERYONE. A too weak character will only make the game boring (or at least harder) for ONE player
- Make then tough instead of strong: If you make them dish out less damage, also make them able to withstand more damage. Some special saving bonuses or custom rules that make them stay in the fight longer can go a long way.
- Fluff! When making a DP for a campaign, make sure the fluff is where you focus. A "rogue" DP should not have strength skills and so forth. However, this should not get in the way of...
- Personal trait: Some DP's may go against the usual fluff by having a certain character kink that makes them stand out. Remember, for instance, the orc crossbow company! Orcs using ranged weapons and chainmail. Very un-orcy, but very colourful characters! As such, you could make an Orc Hunter DP with a big hornbow, or maybe a Marienburg Beggar with nasty clothes, and a penchant for killing rich people. Maybe even a Vampire with necrophobia! As long as it adds to the story of the character, fluff can be overridden!