It depends on how and who is is doing the cutting. Is this for a class, where they are going to be teaching a particular cut so you do not get a choice as to what the cut will be, and the
teacher will be doing the cutting as demonstration? If that is the case, you do not get to choose the cut or length.
Or is this a situation where you can tell the cutter the length you want, and (s)he cuts a style they are learning for that length for his own practice & experience?
I have never been to Ouidad.
I get my hair cut by the absolute best stylist at Vidal Sassoon and pay top dollar for her. She's the Creative Director there and does major celebrities. As one of her regulars, she has always treated me the same as she does the celebs. It's my one big way I treat myself.
Once, when she was out of town, and I wanted another stylist at her level, I was told there is no one else. She's IT. I still wanted my hair cut right away, I decided to get a model cut by one of the students. I figured, it's the same company, same cuts, right? Plus, I'd get to save a boat load of $$$.
What I didn't know was that the students learn from a
video first

and then are supervised (loosely) by one of the teachers - who are off at the other end of the salon doing their own cuts.
I had 4 INCHES I was interested in having cut off, so I figured, no problem, the student has 4" to practice cutting off to get right. But, the teacher told him to cut to the exact length right away and start from there.
Problem was, he really didn't know how to cut. Sassoon has a very particular way of cutting. Sassoon revolutionized the hair world back in the '70's. So, this student kept cutting SHORTER to try to fix his mistakes, from where I already wanted my hair length to be. TWO & A HALF
HOURS later

the teacher finally had to come and FIX my hair on one side. The student was then supposed to try to cut the other side the same way. that took another half an hour.

My hair did NOT match in the end.
My hair was too short to even get it properly re-cut by my regular stylist, who probably could have done a beautiful job. As Creative Director, she actually is the one who fixes problem cuts, when a customer complains or is unhappy about whatever cut they got from one of the pro stylist there. That's how I originally met her.
2 MONTHS later, when my hair grew in enough, I finally went back and paid what I usually do for a great cut. What I learned was never to skimp again on a good cut. It wasn't worth it.
I should have listened to myself.

I almost told him to cut the hair longer, in CASE I needed it re-cut later. I had a 4" margin, I could have used. I almost even questioned the teacher, when she said to cut it to the exact bottom length and then start from there.
In that 3 hour time period, I wouldn't have minded if he had to cut my whole head 3 times over, cutting an inch off at the time, as long as he ENDED with the length I wanted. He shouldn't have
started there.
When I saw that the cutter wasn't cutting the way the teacher had told him, I should have spoken up, "She told you to cut by pulling the hair straight up. You keep pulling it over to the side."

But, I didn't want to give him a hard time, he was so nervous.
But,
I was the one who ended up with the bad hairdo when it was over.
Good luck, I hope you have a better experience than I had. Trust yourself, if you feel things are going wrong. And don't be afraid to leave, if, once it's all explained to you, it all turns out to be not an experience you want. You don't OWE them anything. It's your hair. I don't care how free, or nearly free it is.