modify ds value before written in rrd file

Has anyone tried, or perhaps know how to modify a ds value after it is
pulled via SNMP but prior to being written to the rrd file?  

I have an issue where i pull a value, store it in an rrd file and then
calculate a % by using the scale option under the graph.  It works fine,
but if we ever add more total capacity it will throw the history off
when I modify the scale calculations.  I'd like to pull the value via
snmp, calculate the % and then store the % in the rrd file rather than
just a total used so the history will always be accurate.


############
this is what i have:

datasource i1800
   ds-source  =  snmp://%snmp%/49.3.83.83.55.4.49.56.48.48

graph  i1800
   legend      =   "Total Incoming"
   units       =   "%"
   color       =   black
   y-min       =   0
   y-max       =   100
   scale       =   1.39,* 

############
I'd like to do something like

datasource i1800
   ds-source  =  ((snmp://%snmp%/49.3.83.83.55.4.49.56.48.48)*(1.39)

Or
datasource tot1800
  ds-source = snmp get total avail
datasource in1800
  ds-source = snmp get total in-use
datasource avg1800
    ds-source   =   exec:0:"ca-tt-avg.pl ds0 ds1"   (which would return
an avg to graph)


I'd like to keep as much as possible within cricket to keep the
calculation time slow.

thanks


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