Partial pay for initial pay period

I am salaried, I normally work M-F, I very rarely work weekends.

My employer pays semi-monthly. My start date was two days after the start of the pay period so I missed working two workdays that month. I worked the remaining 10 working days that month plus the day I started employment was a weekend and I worked a full day that day.

There were 16 calendar days in the intial pay period and 12 working days. I worked either 10 or 11 days depending on whether or not you count the initial weekend day that was my first day of work.

My employer paid me 11/16ths of my normal pay. I believe I was entitled to either 10/12ths or 11/12ths of my normal pay. My employer has not responded to my complaints and it has been many months since my incorrect paycheck that I promptly complained about.

They have made the same incorrect calculation with more employees than just me.

Am I correct and what can I do to get my employer to pay me the correct amount.