Video of computing comet close approach using exorb and solex
http://moozoo.dyndns.org/C2013A1/C2013A1Solex.mp4
Using the Extras option (!) you can increase the order of the intergration and enable the use of 80bit floats. It doesn't make much of a difference.
To use findorb to compute the orbit and then use solex to do the intergration you need to use findorb's "Make Ephemeris"
http://moozoo.dyndns.org/C2013A1/findorb.jpg
Take a copy of the C2013A1.SLX file and edit in the findorb values.
To convert to the units used by solex
Take the 1st number and replace the first line in the .slx file with it
Take the 2nd,3rd and 4th numbers and multiply by 149597870.691/1000000 and replace the 3rd line in the .slx file with it
Take the 5th,6th and 7th numbers and multiply by 149597870.691/(24*60*60) and replace the 4th line in the .slx file with it
The 149597870.691 is the AU_TO_KM constant in the findorb source
The result
--------------
2456205.093440
0 0 0 0.0000000E+00 C2013A1
1.3102997512433800E+02 1.1404778167143200E+03 -2.9852251770828400E+02
3.1353047253305100E+00 -1.4621796132689200E+01 -5.2773661621146100E-01
0.00000000E+00 0.00000000E+00 0.00000000E+00 1.11000000E-01
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Solex using a filtered (residuals over 1.5 removed) findorb orbit gave a close approach of 145614 km at 18:25