My library until recently has been all mp3 and I have religiously used MP3Gain to apply a track gain normalization of the default 89 dB to each file in my library.

So now as I have started adding flac files instead of MP3 I would obviously like to normalize these to the same level. I have downloaded and tried Foobar2000 but don't quite get how the replay gain tags work and are calculated/applied. Is there any way I can set the replay gain tag to 89 dB as in MP3Gain?

I believe I could let Foobar2000 look at my whole library and do an average normalization across that, but I am trying to avoid that as 1)it would take a very long time and 2)I don't want to mess up my library I am already happy with and undo the work I have done with MP3Gain.

Has anybody else encountered this? Any ideas/solutions? Thanks!
On-Wall M5HP LCR, QS8 & EP500 in 7.1