You do not calculate indicated airspeed: It is an indicated value, you read it from the airspeed indicator. (I suppose you can calculate it if you really want to, but I've got no idea why you would want to do so in the context of flight planning.)
You determine what airspeed to use for flight planning purposes using your aircraft's performance data (from the POH or other manufacturer documents): There will usually be either a chart or a table giving true airspeed as a function of power setting and density altitude.
(True Airspeed is a more useful number for planning purposes as it saves you a calculation.)
When in flight you can calculate your true airspeed based on the indicated airspeed and density altitude. You can then compare this to your planned figures from the performance charts to determine if adjustments are necessary to your flight plan (ETA, Fuel use, etc.)