What is the amount that I should deduct from my employees weekly salary for disability tax?

Where?

I am not aware of a diability tax. Are you asking about the premium on a disability policy? Workers Comp?

JudyKayTree:

Some states (California and New Jersey, to name two) require an employee contribution to disability insurance. It is designated as SDI on the paystub and is considered a tax.

Clickshot:

Contact the state tax department to determine what percentage to deduct; the amount varies from state to state. Note that there is usually an annual maximum amount that can be deducted from the employee's salary.