First let me express my profound gratitude for considering 100 kT as a "smallish" force as is right and proper.
For both airborne and spaceborne fire support the hit chance is roughly the same as for your ground units, with the same target selection options as the heavy bombardment component. For airborne support the fighter pod types have variable damage/penetration depending on size and tech level so I will leave those alone, however for spaceborne support the effective damage per shot is 20x the square root of the weapon damage and the armor penetration is half this, e.g. for a 10cm railgun (1 damage per shot) we have 10 AP and 20 damage, while a 20cm railgun (4 damage per shot) deals 20 AP and 40 damage. These values are comparable to light and medium bombardment (LB and MB) components at racial attack 10 (which would be granted from researching 20cm railguns), which gives us a rough equivalency for comparisons.
Given a smallish 100 kT of forces on the ground, we might assume for sake of argument that these are 3x 25 kT tank brigades and a single 25 kT support brigade including HQ, supply vehicles, and medium artillery - this 3:1 ratio being implied as optimal by the officer promotions ratio. Assuming that roughly half of the HQ formation is logistics vehicles (rough calculations suggest this is not actually enough to sustain a month or so of campaigning by 75 kT of tanks, but let's assume it is), this implies some 12,000 tons of artillery or approximately 240 MB guns. Thus to bring equivalent firepower would require around ships in orbit totaling some 240x 20cm railguns. Recent behind-the-scenes ship design experience suggests that a reasonable amount of railguns of this size is perhaps 10x per 10,000 tons of ship, which could be more or less depending on your engine and armor policies, so to provide equivalent firepower to a 12 kT artillery force you need some 240,000-ish tons of naval vessels in orbit. If you like, reduce this to 180,000 since railguns fire four shots while artillery fires three, but the order of magnitude is not changing significantly here.
On one hand this seems quite inefficient - as it should be, ground units should be the most effective at ground combat particularly as they do not have much else to be good at. However, orbital bombardment does have a few advantages. On one hand it does not consume expensive ground-based GSP, maintenance supplies are not usually a serious bottleneck provided that a space empire has sufficient sources of duranium and gallicite particularly when the guns are only fired once every eight hours (effectively). By contrast GSP requires relatively scarce ground unit BPs to produce so is often a key constraint especially for an unprepared force. On the other hand, naval weapons need not be purpose-built for ground bombardment and you can simply loop in a spare battle fleet to shoot at tiny men on the ground if they happen to be in the area. Additionally, in the absence of STOs naval fire support cannot be killed by the enemy ground units unlike the artillery on the ground, so as a fight goes on the naval support element becomes progressively more powerful relative to the battle-damaged ground units and the contribution may be not insignificant.
I have focused on the fire support method of naval bombardment here, however there is also the option to bombard enemy ground units without coordination with the friendly ground units. I'm unclear on whether this happens every ground combat increment (8 hours) or at the weapon rate of fire; if the latter this is a fairly quick, though inaccurate and correspondingly MSP-expensive, way to reduce the enemy ground forces, though I think the collateral damage is likely to be much worse as each shot has a 1/3 chance to cause population damage instead. From the wiki I am not clear on if fire support operations cause this damage.
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I am using maximum tracking speed. I think I reduced the range of the BFCs because you get pretty sharp diminishing returns on range when firing at 10,000km -- going from 64,000km to 100,000km only changes the range multiplier from 0.84 to 0.9 while increasing the size of each BFC by 56%. But you're right that PD is probably not something I want to skimp on.
If you want to do math about it, the optimal value of the BFC range is going to depend on how many guns you have per BFC. If you have, say, 100 guns per BFC, then even a small improvement in BFC range will give a significant improvement in kill rate. Whereas if you have one BFC for a handful of guns, reducing the BFC size may allow you to fit another gun instead which will probably give more expected missile kills than a modest range increase. There is also the practical question of fitting a BFC to a specific ship class - you might be able to gain a few points of missile kill % by taking off 20 tons of fuel and pushing up BFC size by a small amount on one ship class, but maybe another class does not have that kind of room.
Generally I stick with 1x range multiplier for PD BFCs and don't think about it anymore.
