if you are waiting to measure approach until you have full load then you are missing a lot of information.
chiller is running at 50% RLA and the evaporator approach is at 1F...good or bad?
if you can't judge it until the chiller is at full load then you are not giving a clear picture to the customer...btw...good...why? well, we know that approach temperatures go up with load. if the chiller is at 50% and 1F approach, then it will most likely be ok when it loads up to 100%...it may get as high as 4-5F in this case.
chiller is running at 50% RLA and the evaporator approach is at 5F...good or bad?
since we know that the approach is going to go up when it loads up, then we know that this chiller is going to probably have a 8-10F approach at 100%...this is bad, however, we know this long before the chiller is at 100% and we can take action to prevent this.
know how to read your approaches!