The MX60 can't boost the voltage , so using 3 panels per string vs 4 isn't a good idea.

What can happen is in hot weather , the true mppt point could be BELOW the battery charge point. This is the only real knock on mppt controllers (added cost but in hot weather , not added performance).

You , by chance are doing EXACTLY what is necessary to get the mppt advantage ALL the time , by adding another panel to your string , the mppt point is always above the charge value for a 48 volt battery bank , so no need to change , you have it right as it is !!

Thanks for the responses! I forgot about the MPPT possibly being below the batteries.

The "edge of cloud effect" is something I would have never believed or figured out. Thanks again for the valuable info.

Anthony

heres something i was reading off the xantrex site, about using pfc caps to correct the inverter output
http://www.xantrex.com/web/id/344/DocServe.aspx

also this
http://www.xantrex.com/web/id/343/DocServe.aspx
if i understand this right,the inverter output at the meter may not be correct due to a power correction factor due to inductive and reactive loads.
i hope this helps