When I was running DH, the combat balance point I looked at was average PC's T, Torso AV, and primary weapon damages and pen's in ranged and melee. Used 5.5 per die, rounding up at end, then multiplied by total number of PC's.
If the difference between average damage+Pen and opponent's T+AV is significantly different between sides, the battle will tend to be similarly lopsided.
EG: PC's average Toughness is 40... so T=4. Average torso armor was 4. Weapon damage was averaging 8 (1d10+2), with Pen 4.
NPC side had T6 and AV8, average damage 7 (1d10+3), pen 0...
PC Side was 8+4 vs 6+8, for -2
NPC side was 7+0 vs 8, for -1.
Now, it has to be shifted to positives; move both up the same amount so the lower is +1. In this case, PCs 1, NPC's 2. Now multiply by the numbers. I had 7 PC's, and used 4 NPC's... for 7 and 8. Then multiply by average skill as a percentage... PC average was 45, NPC was 33, for 7*0.45=3.15 and 8*0.33= 2.64. It's clear the PC's have an advantage, but a very weak one... and since in the initial stage, both were negatives, it's going to involve a lot of armor staging.