I have a novel in progress where there is currently about three chapters of flashback material. Now, one of those chapters could very well be moved to the opening chapter, as I feel it's strong enough. However, the other two I'm pretty much stuck with. This isn't much in terms of volume - maybe 6,000 words or so - but I'm concerned about the reader getting thrown off the horse so to speak, as the main story is much more engaging and fast-paced. Also, how soon is too soon to insert a flashback of this length, in your opinion?