No your don't ever reduce the value of the FL...what happens is...
If the hindrances are soft they only count on the initial shot (so you fire through 2 grains its +2 for that...then likely -1 for FFNA)...
If the hindrance is hard (like orchard say) then you add these as DRM to the attack, but the base value of the FL remain as is....so you had a MMG FL down already...so 2 FP FL....but now an OPFOR MMC moves through the lane, and between it and your MMG are 2 orchards and a grain hex....you will add +2 DRM (one for ea. orchard)...the grain has no effect now the FL is in place...then likely you will -1 for FFNA...leaving a net +1 DRM on the 2 FP col...