Do you two have significant wealth-related income deductions such as mortgage interest or margin loan interest? Or is this really about insufficient evidence? The formula according to Kt. ZH is
Ertragsminderung = A x B : C
A: Schuldzinsen und Unkosten
B: Steuerwert DBA-Titel
C: Wert Total Aktiven
And then
Maximalbetrag = (Bruttoertrag - Ertragsminderung) x Anrechnungstarif
“Anrechnungstarif” is your average (not marginal) income tax rate.
In Kt. ZH they send you a “Berechnungsblatt Pauschale Steueranrechnung” with all these values if their calculation differs from your DA-1 request, which makes it easy to comprehend.