Just how Deadly is Deadly?

There's a chart here valid for SE5:
http://wiki.spaceempires.net/index.php/Facilities_%28SEIV%29#Climate_Control_Facilities

It also explains how the Climate Control facility works. However I wouldn't be surprised if a planet with 0.0 for conditions never improved as 0 * % will always be 0.

Space Empires Depot | SE:V Balance Mod

Actually you should check how it does rounding. if it always round up then there's no problem.

I think it may truncate, seen that allot in the scripts when decimal numbers where used.

The event is possibly in 10ths of a percent, rather than whole numbers.

SpaceEmpires.net | Space Empires Wiki

Right, and the game might be treating the value as a tenth (or even hundredth) of a percent, rather than a whole number. Refer to how chance values in StellarAbilityTypes.txt function, for example.

SpaceEmpires.net | Space Empires Wiki

This is fixed in version 1.62 as the improvement of the climate control facility is now additive not multiplicative.