Soundcheck problems using different versions of iTunes

I managed mp3 library imported in iTunes with Windows XP and iTunes 8.0.2.20 till last year. I always verified the Volume adjustment of each mp3 and the correlated hex values written in the field iTunNorm (using mp3Tag).
Last year I updated to Windows 7_64 and to iTunes 10.6.3.25 and went on importing new files in iTunes.
With some test, (same file imported using iTunes 8 and iTunes 10 on different PCs) I realized that:

1. iTunes 10 don't write iTunNorm tag anymore in the file mp3. I have a Volume adjustment, but with mp3Tag I can’t read any iTunNorm field with the famous ten hex values.
2. iTunes 10 and iTunes 8 use a different way to calculate Sound Check; Volume adjustments calculated with iTunes 10 are lower of about 1,5-2,0 dB in average against iTunes 8. But sometimes the difference may be very high (-5 dB or -7dB for example in iTunes 10 versus iTunes 8).

My questions:
i. Where iTunes 10 write Sound Check info to the mp3 file and how can I read it?
ii. What can I do to avoid the discontinuity in Sound Check values due to the use of iTunes 8 and iTunes 10 without resetting all data?
iii. What is the best: SoundCheck of iTunes 8 or SoundCheck of iTunes 10 (which is the difference)?

Thanks in advance for your answers.