Go to:
YourDrive:\SteamLibrary\steamapps\common\ICBM Escalation\speed.txt

You will see smtn like this

Speed 1 2 4 16
Divider 1 1 1 1 // ignored if the speed is > 1

ReplaySpeed 1 5 20 2000
Divider 1 1 1 1 // ignored if the ReplaySpeed is > 1

Its simple fraction
Divider is the denominator
Speed/ReplaySpeed is the numerator

for instance if you want half speed at speed lv1

Speed 1 2 4 16
Divider 2 1 1 1 // ignored if the speed is > 1

ReplaySpeed 1 5 20 2000
Divider 1 1 1 1 // ignored if the ReplaySpeed is > 1

the example above makes :
Speed/Divider = 1/2

Now go nuts and nuke the world