Grain absorption is roughly a one to one ratio,take into account your deadspace loss which includes trub and liquid below your valve outlet and then if you're using a plate chillier you need to account for this plus hoses attached.
Fill your boiler/brew kettle to full volume and boil with just water and after an hour cool and drain as normal and drain to your FV and measure your losses.This will give you boil off/deadspace and chiller loss.
With this done you've just calibrated your equipment and add it to your recipe calculator and then allow for extra grain you'll need to add for your losses and get your target FV level.