In case this helps anyone else I finally got this to work by including the tab reference in each part of the formula like so:
Original formula:
=IF($I2<>"D/E",$J2+($K1-$L1),$J2)
Working formula:
=IF(FixedQuotaGuide!$I2<>"D/E",FixedQuotaGuide!$J2+($K1-$L1),FixedQuotaGuide!$J2)