So basic engine/idle theory... **assuming your trigger1 offset is correct**
engine power output is from compressing & igniting fuel + air. The more fuel + air you have in the cylinder, the more potential power. The ignition angle (ie "when" you ignite the mixture in the 360* cycle) determines how much of that potential power is actually created. In 95%+ of petrol/ethanol engines, you "control" the air intake, and the computer/carb adds an appropriate amount of fuel to match.
To have a stable idle, the engine output power (torque) must equal the amount required to keep the engine turning over. Any less & it stalls, any more and the revs increase.
So the 2 levers you have to pull to get a stable idle (at any rpm) are
1) how much air you let in
2) what ignition angle you fire the spark plugs at
You are only changing #2 here, but it sounds like you have #1 a bit off vs the factory specs. 25* ignition at idle is quite high, so I suggest you need to increase the air flow at idle - either idle stop screw on the throttle, or increase the idle control settings in the ECU to allow more air. You will then see your idle jump up to higher rpm, and when you reduce the ignition table values it should come back down. I cant give you exact values, but typical factory ignition idle angles are usually in the 10-20* range, and in your case it sounds like the manual says 5*. You dont have to match this exactly as your idle numbers can also differ from factory, but its usually a reasonable starting point.
You are extremly unlikely to have knock or other spark related damage at idle speeds, so there is no real "dangerous" ignition number for idle, its just what works well for your car. Knock only becomes an issue as the engine comes under mid-high load (note that low rpm can still be high load however when you press the throttle hard)