I’d recommend taking the guesswork out of it. A Lux meter (33 buck on Amazon) really comes in handy for determining how much HID light your plants are getting. I think your way too close. I just tested mine under 750 watts and I’d do 30 inches.
An inexpensive lux meter lets you measure that amount of light at a specific point in space - learn how to use this tool to increase your yields!
www.growweedeasy.com
Dr.Meter LX1330B Digital Illuminance/Light Meter, 0 - 200,000 Lux Luxmeter
https://www.amazon.com/dp/B005A0ETXY/ref=cm_sw_r_cp_tai_M0khEbJDSFPWV