I've yesterday used my 80-200 nikon lens for thd first time with ekos and just used 200 mm focal length. Actually decreased it to 135-ish and this was really quickly solved in the 'align' tab.
Just 'capture and solve' with 'do nothing' selected showed a FL of 136,4 mm.
This you could then use for your focal length.
I wonder which one is actually stored in the fits header. Nominal or actual FL?