Outfall Invert Level in the Centre of Storage.

We are trying to add storage off of a manhole, however don't want the IL as per the manhole or the base of storage.

We have designed the soakaway 10m x 20m with a depth of 1m. We want the IL to be different to the manhole IL and have added the soakaway IL higher at 62.60.

However, we don't want the soakaway IL to be the base, but half way up the 1m. How do we model this?