With higher levels, one of two things could be limiting:
1) The level of fungal alpha-amylase or
2) The Glucose Determination Reagent.
We suggest the following: first terminate the fungal alpha-amylase reaction with 8 mL of acid as per method; then dilute an aliquot four fold in 0.2% sulphuric acid; then analyse this and allow for dilution in calculation. If the absorbance is above 1.20, the solution, after alpha-amylase treatment should be diluted 2-fold and the AMG step repeated. If a dilution of much more than 2 fold is required, then you should use less starch in the initial step (say 50 mg).
We have measured samples up to 58% Starch Damage successfully with the K-SDAM method (scientific paper can be provided) without any issues.