120' per 1 degree C different from ISA tends to work well.
In your example, 30C at 5500ft is ISA + 26
so 5500 + (26 * 120) = 5500 + 3120
DA = 8620'
Just noticed you asked for a quick way to calculate ISA deviation too. I am not sure if there is a 'quick way' but temperature should reduce by 2C per 1000' above sea level. so take your altitude and multiply the thousands by 2 and take the difference from 15 and that's the ISA temp. then you can just work out the difference.
Again using your example, 5500ft with 30C
5.5 * 2 = 11.
15 - 11 = 4 so ISA would be 4C
Actual temp is 30 C which is 26 more than 4, hence ISA + 26