I advise you to ko lugia and come back after you beat the e4 and went to kanto.
When you do that, go back to lugia cave and save 1 step before you enter the room lugia is. When trying to hit your seed, the first couple of seed that you hit that isnt your target frame, input that seed in the rngreporter and capture lugia. Match the IV of the lugia with the current seed. Doing this will tell you how much the frame advance between entering the cave and encountering lugia. IMO it is best to kept lugia frame including in the calculation
It best to do this 2 or 3 times. When you sure you know how much your frame advance, try to hit your seed, but keep in mind of the frame advance.
[if you did this 3 times and all 3 result showed the frame had advance to frame 6 from lugia frame(which is frame 1) + the amount of frame from entering the cave(which would be 5 in this case) then your formula should be:
target frame - 9 (for elm calls) - 6(for lugia frame + frame advancement) - ( # of the frame advancment via radio or elm)