As an AHT you are paid on the Leadership Scale. Therefore, you are paid to lead. Whether this is as an AHT or as an Acting DHT, you have presumably accepted and been given that responsibility. 110% has nothing to do with it. Try 100%. Nobody will ask for more. To be honest, if you are not prepared to step into the breech when the DHT takes maternity leave, then I suggest that you step down from any leadership responsibility and go back to being a classroom teacher.