You mean like from a UAV?
TT
The UAV should serve as a spotter, and follow those rules for correcting fire. I was thinking more along the lines of radar guided fire, where the radar tracks the round and adjusts before the round has landed. Part of it would be a fixed tonnage composed of the radar guidance and Fire Director, and part of it would be the tonnage per gun to allow for better control. A larger fixed tonnage system would mean less tonnage needed on a per-weapon basis, but the higher fixed tonnage means
An interesting idea would be allowing multiple sets of the fixed tonnage components to allow for primary and backup Fire Directors. You need 1 ton of Fire Guidance per X weapon tons, and Y tons for a Fire Director. If you use different generations of Fire Director (i.e. a high-quality primary and a lower quality secondary), then figure out what is the highest tonnage needed per weapon ton, and allocate that.
I.e. Fire Director version 3 needs 1 ton of weapon guidance per 6 tons of weapon tonnage, while Fire Director version 2 needs 1 ton of weapon guidance per 5 tons of weapon tonnage. The second one requires more tonnage per weapon, so that is the tonnage needed.
If you wanted to further include regular Targeting Computers that need 1 ton per 4 tons of weapon, that tonnage would be the new requirement.
So for 600 tons of weapons, the options are:
Fire Director Mk 3: needs 100 tons of on-mount weapons guidance, plus some amount for the Fire Director
Fire Director Mk 2: needs 120 tons of on-mount weapons guidance, plus some amount for the Fire Director
IS Targeting Computers: needs 150 tons of on-mount weapons guidance, but no Fire Director needed
If you wanted just the Fire Director Mk 3 as primary and secondary, you would have the two Fire Directors, and just need 100 tons to control the weapons (as the Primary and secondary Mk3 can use the same on-mount weapons assistance). If you wanted a Mk 3 as primary and the smaller Mk 2 as your backup, you would need a total of 120 tons (the Mk 3 can use the Mk 2 aiming systems). If you wanted your weapons to still be able to operate even with just local control, you would need 150 tons for the Inner Sphere Targeting Computers, as both the Mk 3 and Mk 2 could use them.
There would be no stacking of targeting bonuses, you would only use the highest bonus. So if you had 150 tons of Targeting Computer (providing a +1 to-hit), a Mk 2 Fire Director (providing a +2 to-hit), and a Mk 3 Fire Director (providing a +2 to-hit), you would only get the +2 to-hit. If the Mk 2 gets damaged, there is no degradation of to-hit (since the Mk 3 is providing the same bonus). Once the Mk 3 Fire Director gets hit, the target bonus gets reduced to the +1 (from the existing Targeting Computer).
(To me Fire Directors would have a tonnage range where they are optimal, but above and below that range other Fire Directors have the tonnage advantage. I.e. if Fire Director 2 massed 10 tons base and needed 1 ton per 5 tons of weaponry, it would only be better tonnage-wise than regular Targeting Computers when it is controlling over 200 tons of weapons. If Fire Director 3 is 100 tons base and 1 ton per 6 tons of weapons, it becomes superior tonnage-wise when there are over 2700 tons of weapons. I needed to make Fire Directors better than just putting Targeting Computers on everything, thus the '+2' to-hit. The order of magnitude was to make the math easy. Increasing the bonus for each Fire Director didn't seem to fit, so I just left them all at '+2', instead of having the Fire Director Mk 3 give '+3' to hit)