You should have no problem with 1 foot (30cm) or lowe
Your problem is not going to be the amount of light it produces but the amount of heat it produces. At 36w, assuming 5050 led will produce about 114f or 46C at source.
So just watch the temperatures. Lower until it gets to hot.
If they are just deflasked, then start at 12 inches (30cm) or higher then slowly lower it over the next few weeks.