To get almost precise (3% error) SoM or SoT, you need to send 20% of your thieves.
SoM has troops out exact no matter how many troops you send

You have two choices:

1) you get a 20% SoM and oversend by 7% (3% error, 4% randomness)
2) you get a 20% SoT and a 1 thief SoM and you substract the number of elites to get defense

If you wanna work with option2, you can use angel as described above, but you can also create an excel sheet (takes 5 mins) or just calculate it without any tools. The way it is done is simple, you figure out the defensive military effectiveness from the SoT numbers, then substract the ElitesOut*EliteDefense*DefensiveME from the SoT defense at home. Then add 7%.