Essential: Obtain the Schleicher ASK-21 Flight Manual in english.
It will be specific to your serial number.
Note the manual revisions listed at the Schleicher website.
If you attach the spin weights, consult the recent manual revisions.

Optional: A calculator but only for a specific ASK-21 in Nevada: http://soaringnv.com/WeightBalance.php

Best to develop your own. Consult your A&P / IA.