CNC setup - setting accurate number of counts per unit..

This may be a dumb question, but I have converted my mill to CNC,
and ball screws, and it all works fine.... I'm not sure however if
it is accurate or not.

I tried to set up the CNC my moving 1.0" per a dial indicator and
then adjusting the software counts to match. When I put a tape
measure on the table and move the full travel, its off a bit... How
do I get it set accurately? apparently a 1" travel dial indicator
isn't enough, and I'm not comfortable 'eyeballing' an tape measure
over the 12 and 24" of table travel..

Thanx.