Natural gas option selling

Hello,

I am building a strategy for Natural gas options. I am selling Natural Gas ATM Call and Put options at 4:30 pm every day. I am set limit order as market order will not execute…

I am using round Math Operation formula. I am getting issue regarding place ATM order. For example, if NG price around 151 still my order place at 155 insted of 150 strike.

What formula should I use?
What is mistake in my formula?

Anyone have idea what to do?
Please Advise would be Appriciate.