The short answer is that I blindly de-ice (your term) if there is anything slush like, slurpee like, or frosty like on the wing, or if I think something may develop. Some of my considerations are temp, trends, wind, moisture on runway, fuel tank temp, and time from closing the door to takeoff. I especially deice if I have done a quick turn and it is below 10c, as the fuel will almost always be colder than -10.
The law for the G550 is as follows:
01-30-10: General
Icing conditions exist when the Static Air Temperature (SAT) on the ground
and for takeoff, or SAT in flight is 10°C (50°F) or below, and visible
moisture in any form is present (such as clouds, fog with visibility of one
mile or less, rain, snow, sleet and ice crystals).
Icing conditions also exist when the SAT on the ground and for takeoff is
10°C (50°F) or below when operating on ramps, taxiways or runways
where surface snow, ice, standing water or slush may be ingested by the
engines or freeze on the engines, nacelles or engine sensor probes.
Takeoff is prohibited with frost, ice, snow, or slush adhering to the wings,
control surfaces, engine inlets, or other critical surfaces.
A visual and tactile (hand on surface) check of the wing leading edge and
the wing upper surface must be performed to ensure the wing is free from
frost, ice, snow, or slush when the outside air temperature is less than
42°F (6°C), or if it cannot be ascertained that the wing fuel temperature is
above 32°F (0°C); and
· There is visible moisture (rain, drizzle, sleet, snow, fog, etc.) present;
or
· Water is present on the wing; or
· The difference between the dew point and the outside air
temperature is 5°F (3°C) or less; or
· The atmospheric conditions have been conducive to frost formation.