Well, I don't know the plot of your work, so I can't give a rating for you.
The best thing for you to do is to look over your plot & ask yourself questions such as "Is there fighting in it, & if so, how violent is it?" "Is there any romance inside, & how strong is this romance?" "Is there going to be inappropriate language usage?"
Try doing that, & see if you can get a rating based off of the answers you give yourself. Also, you don't "have to" use Serebii's rating system - the one you use works perfectly fine here.
The best thing for you to do is to look over your plot & ask yourself questions such as "Is there fighting in it, & if so, how violent is it?" "Is there any romance inside, & how strong is this romance?" "Is there going to be inappropriate language usage?"
Try doing that, & see if you can get a rating based off of the answers you give yourself. Also, you don't "have to" use Serebii's rating system - the one you use works perfectly fine here.