Actually, on average each critical does slightly more than that.
Whilst probability is only ever a guide (especially where beams are concerned!) the rules are as follows:
Hits
Regular AD: (7-x)/6 chance of a hit per AD where x is target's hull.
AP/SAP/Weak weapon: As above but with modified hull value.
Twin-linked: As above but with 35/36 for 'effective' hull 2, 8/9 for hull 3, 3/4 for hull 4, 5/9 for hull 5 and 11/36 for hull 6.
Mini-Beam: 1/2 a hit per AD
Beam: diminishing series - chance of x hits is the chance of getting one hit less times by 1/2, so the series goes 1/2, 1/4, 1/8, 1/16. A chain of 1/2+1/4+1/8+1/16 adds up to 1 to sufficient accuracy if you go on long enough (have a try on a spreadsheet if you don't believe me), so a beam should average out at one hit per AD.
Damage Type:
Energy Mine: 1/6 hits are Bulkhead, 5/6 hits are Solid
Non-Precise: 1/6 hits are Bulkhead, 2/3 hits are Solid, 1/6 hits are Critical
Precise: 2/3 hits are Solid, 1/3 hits are Critical
Damage:
Bulkhead: Single damage gives no damage. Double or triple gives one, quad gives two.
Solid: Pretty obvious - one damage times the damage multiplier.
Critical:
There are 36 combinations of equal likelihood assuming unbiased dice for the critical 'type' and critical 'severity' rolls - although some of them are the same (critical 'type' rolls of a 1 and a 2 are identical, for example).
Assuming you take one of each possible roll, as well as suffering more system dropouts than you can shake a pointy stick at, you take an additional 59.5 damage and lose 73.5 crew - criticals are slightly more benevolent to regular damage than to crew score because of (a) critical table '4' and (b) the 'exploding console' syndrome which invariably accompanies systems failure in science fiction shows.
Averaged out over those 36 hits, that makes 1.7 damage (to two decimal places) and 2.0 crew loss.
In additon to this each critical does the normal 1 damage and crew loss of a solid hit, for a total of 2.7 damage and 3 crew loss per single damage critical, times any damage multiplier.
Obviously you also need to factor in stealth, dodge, GEG, etc at appropriate points.
So the Transport firing on a Sho'Kos:
Discharge Gun
2 AD fire
2 Hits (beam)
Sho'kos evades 1/3 of the fire with dodge 5+ (about the only means of defence that works here!) leaving 1.3333 hits
1.3333 hits translates to 0.8889 Solid hits and 0.4444 Criticals due to precise.
Double Damage means that each solid hit does 2 damage/2 crew and each critical does 5.3 damage and 6 crew.
Total= 4.2 damage, 4.4 crew
And the Sho'Kos firing back:
Burst Beam
1 AD fires
1 hit (beam)
1/6 bulkhead hits, 2/3 solid hits, 1/6 critical hits.
bulkhead hit does no damage, solid hits do 1 damage, criticals do 2.7 damage. No crew score to attack.
Average of 1.1 damage. In real terms Adaptive Armour drops this to 1 - if it's ever enough to round up to 2 then it's dropped straight back to 1.
Medium Pulse Cannon
2 AD fire
2/3 of a hit due to hull 5
0.111 bulkhead hits, 0.444 solid hits, 0.111 critical hits.
bulkhead hit does no damage, solid hits do 1 damage, criticals do 2.7 damage. No crew score to attack.
Average of 0.74 damage - not enough for Adaptive Armour to affect.
Light Pulse Cannon
4 AD fire
4/6=2/3 of a hit due to 'effective' hull 6 (weak weapon)
0.111 bulkhead hits, 0.444 solid hits, 0.111 critical hits.
bulkhead hit does no damage, solid hits do 1 damage, criticals do 2.7 damage. No crew score to attack.
Average of 0.74 damage - not enough for Adaptive Armour to affect.
Total of 1+0.74+0.74 = ~2.5 damage per shooting phase.
Transport will self-repair 1 damage per turn after all opponents have a chance to fire.