There are two schools of dealing with the LFE channel on the mixing end, so there could be two good answers to this. It will depend on the material you have in hand.
Either the mixer removed the bass from the full-range channels and sent it all to the LFE himself or he mixed for bass-managed systems by leaving some bass in the LCR channels so the end-user’s amp routes that bass to the LFE through its bass management system. The first option has a lot more traction these days, but I still see some mixes with quite a lot of bass in the full-range channels from time to time.
If you listen to the individuals L, C and R channels and there is some material below 250Hz, you could probably drop the LFE channel. If those channels don’t contain bass material, you should include the LFE in the downmix. In that situation, -10dB should be good.
IHTH,
JC Boulay
Technical Director
Audio Z
Montreal, Canada
http://www.audioz.com