Then mod init to what suits your taste.
Open descr_campaign_db.xml in the data-folder, I suggest you use the freeware Notepad++ but guess normal Notepad can work too.
Go to this section:
Code:
<spy_base_chance float="40.0"/> <!-- base spying chance -->
<spy_level_modifier float="0.15"/> <!-- modifies the amount the subterfuge rating of a spy affects chance of success -->
<not_spy_level_modifier float="0.5"/> <!-- modifies the chance of success if spying object not a spy -->
<spy_public_modifier float="1.0"/> <!-- inversely modifies the amount the public security aspects of the target affects chance of success -->
<spy_counter_spy_modifier float="1.0"/> <!-- inversely modifies the amount the combined subterfuge rating with the target affects chance of success -->
<spy_distance_modifier float="1.0"/> <!-- inversely modifies the amount the distance in movement points from the target affects chance of success -->
<spy_secret_agent_target_modifier float="0.5"/> <!-- modifies the chance of success if an assassin or spy is travelling with a target army -->
<spy_sedentary_turns_modifier float="0.2"/> <!-- modifies the amount of turns a target has been stationary affects chance of success -->
<spy_alliance_modifier float="1.5"/> <!-- modifies the chance of success if the target is allied with spy faction -->
<spy_target_engaged_modifier float="2.0"/> <!-- modifies the chance of success if target is in engagement, under siege or battle -->
<spy_in_settlement_modifier float="5.0"/> <!-- modifies the chance of success if spy in settlement they are spying on -->
<spy_watchtower_modifier float="2.5"/> <!-- modifies the chance of success if spying object is a watchtower -->
<spy_in_own_region_modifier float="1.2"/> <!-- modifies the chance of success if spy in in their own factions region -->
<spy_chance_min int="10"/> <!-- min clamp for spy chance -->
<spy_chance_max int="90"/> <!-- max clamp for spy chance -->
<spy_rescale_chance bool="false"/> <!-- flag to indicate if the final chance is non-linearly rescaled to improve spying chance of success -->
As each line has a description it should be easier to know or figure out what to change to balance it as you want.
EDIT: Shall be saved game compatible AFAIK, making it easier to test (though with knight's mentioned restriction in mind)