The dB scale is <ahem> a LOGARITHMIC PROGRESSIVE SCALE... (man that's fancy) as opposed to a linear scale, it's like the Richter scale in that every step forward in the scale is a multiple of the previous number. Every 3 dB higher is actually double the energy/signal, every 10 dB is 10 times, 20 dB is a 100 times the signal strength, so on and so on. When an isotropic point source is radiating 360 deg. in free-space it exhibits 0 gain... so when you see " x# dBi" it's in reference to a number a certain multiple times above that. I'm more used to "dBd" which uses a single dipole as a reference point. The difference in dBi vs. dBd is, uhm, 2.14 or something close to that; 5 dBi = 3 dBd. SO ANYWAY... the difference in gain values between 19 dB and 24 dB is 5 dB, so roughly 2.6 times as more signal strength. Sorry this is so long, but this is what I do for a living.
'Cheers

Chris