The book % filter is the same value as that shown at the top of the Betfair website interface.
If a runner is not included or the liquidity is poor then you will likely get figures not close to 100% which would be standard for a Win market.
The Lay side % book is usually under 100%.
The place market book % can be a lot higher, maybe 300% so you may need to adjust this filter to get bets sent in. 2 places will be >200%, 3 places > 300% etc.
You can see the Book % in the software when selecting each market
