Base: 32 Level: 58 Fel stamina x2
Imp BLood Pact, Sta: 227 Int: 291 Boost: 109
Void Sta: 308 Int: 158 Boost: 101
Succubus Sta: 308 Int: 158 Boost: 101
Felhunter Sta: 308 Int: 158 Boost: 101
Felguard Sta: 308 Int: 158 Boost: 101 --- Fort 54 Arc int 31 Boost: 118
Given its functionality, it's not a large bonus by default but when considering in 500+ stamina, 350+ intellect and raid buffs on top of that the bonus damage adds up quite quickly. With 15 points being attributed bonus to your pets stats per 45 you gain (roughly 1/3) buffs like Fortitude become all the more nice.
79 on pet, 79 on you (yielding 90 with Demonic Embrace), resulting in 100'ish +sta on pet which then yields +7 damage from that one buff.
+130 damage from DK rated against a flat 2,000 damage shadowbolt (for DT/Ruin spec specificly) means it's giving you 110.5 extra damage on the bolt, or +5.525% damage. Doesn't compare to Shadow Mastery at 1k+ spell damage too well, but if you're DT/Ruin spec, it's still a hefty and appreciable bonus.
Fixing formulas
Fel Stamina is 5% health not 5% stamina for the lock, and the values are multiplicative, not additive.