putting random numbers
Defense before your hits from accurate SoT 100k. Defense home from SoM before any hit 70k. Based on what i wrote before defense out is 30k (modified). Lets assume initial OME was 110% and DME 125%. (from accurate SoM). Based on that 70k *1.04 breaks
You do 2 hits, some defense dies and DME raises. We dont care how many died and lets say DME raised by 5% (which is too much btw). When you get an accurate SoT lets assume it says 90k.
Now it interests us what happened with army out. On first SoM raw def out was 30k/1.25=24000 after the hits the defense out would be 24k*1.3 (new DME)= 31200
Now if you had a som it would show that you need 90k-31200= 58800 (*1.04 off course)
With my sugestion you will send 90k - 30k (def from first SoM) = 60k (*1.04 again)
You are oversending a little but you cant fail and because of the SoT every factor is considered (TW less forts maybe etc)
edit
This can fail is if you alter the defense out after the 2 hits. That can happen by ambush or NS. In that scenario the defense out will be smaller that the one calculated at start and you risk a bounce. The second scenario that it might fail is if your target runs huge amount of forts and his DME drops a lot from the hits.