Using my new 1500 Chargemaster  combo to make up some test loads.

It seems the drops are on the high side. When I weigh it on my RCBS 750 its about .3 - .5 over. The same drop is about .2 - .4 over when I weigh it on the 1500's scale. Both scales have been calibrated.
Which one should I trust?
Have tried to under program the drops, but they are still inconsistant between the 2 scales.
Looking for accuracy  to develope loads.
Any advice/tips/suggestions?

I would contact RCBS. Even if you are out of warranty, they may be able to fix one or both of them for a reasonable fee. Just out of curiousity, have you tried to weigh the calibration weight from a scale on both of the scales? Is one of them right on with that weight?

If you have fluoroscent lighting overhead where you are using the scale that is likely to cause your issue.  Otherwise make sure you don't have any drafts (ex. open door or a/c vent,....)  

The 1500 is brand new, got it friday.
The 50 gram calibration weight is the same on both scales.
cant figure it out

No flourescents, have high hats.
Wouldnt the scale door on the 1500 protect it from drafts?

When I received my Chargemaster Combo the static build up on the plastic parts would not allow me to get a consistent reading.  Closing the plastic breeze protection door would throw the reading off by a few tenths.  It was necessary to wipe all the plastic down with dryer antistatic sheets.  No problem after that and it measures exactly the same as my Dillon scale with the same weights.  Can't say if this is your problem but RCBS tech support is very good.
I often see the comment that fluorescent lights will cause a problem.  I have a fluorescent light fixture about 4 feet over my bench but it has caused me no problems for the Chargemaster or the Dillon.

Are you removing the powder pan before turning it on to calibrate?

How level is your scale?