Since nobody has responded, I’ll jump in head first.
Bear in mind that I have done a lot of chromatography ( tlc, hplc) but NO radio chemistry.
In hplc, the criterion for reliable quantitative work is that the minimum signal to noise ratio of a peak should be at least 10:1. Much more accurate if you can get 100:1 or 1000:1.
On that basis, I would measure the background counts of a presumed clean area of the chromatogram for say 30 secs, and then repeat 30 secs on the compound of interest.
Vary time as necessary and Please let us know your outcome!
Regards,