So your running at 20psi, lean and with a difficult to setup 5th injector? How you haven't destroyed your engine yet is totally beyond me. Take the advice offered freely here.

Lower your boost, as much as possible using the waste gate. Try to achieve reasonable fuelling with a boost level of less than 14psi BEFORE trying to achieve good fuelling at 20PSI. Don't walk before you can run.

20 PSI will mean the transient area you are probably struggling to tune will be of very short duration, you won't be able to twiddle your little pots fast enough or with enough accuracy to even begin to tune for anything less than full boost or more than cruise. Life is much simpler on a dyno where you can induce a constant load and hold it until you have achieved the correct AFR, you have a lot of time at specific load points. Unfortunately tuning on the road with out being able to take data logs whilst you have to adjust wanky potentiometers is going to be a long case of trial end error as in most cases it is impossible to accurately maintain or return to a specific load point.

Hence, low boost. Do a run and watch your afr... adjust 5th injector.... do a run and watch afr... adjust 5th injector

This makes sure its all working as you expect and give you an opportunity to learn how to adjust it properly in a slightly safer (for you and the engine) environment.