A good rule of thumb:
Place the butt in the bend of her elbow and have her run her arm up towards the trigger. Measure the difference between where her trigger finger is and the trigger. Add an inch for her to grow into.
Keep in mind if it is a hollow stock you may have to deal with the loss of the means to attach the buttplate.
Shortening a stock is not an easy, straightforward task as it may appear. The angles can bite you in the keester.

Give me a holler if we can be of assistance.