Setting my own exact margins

I'm sure there is an easy way to do this but I can't figure it out. I'm attaching a screenshot of a template created by a photo book company. Images and text should not extend beyond the pink lines--it is the safe art boundary. I added my own safe art boundary in the form of turquoise lines.

However, I cannot seem to make them equidistant from the red bleed lines lines. In other words, I want these turquoise boundaries to act as the pink ones do. I realize that the distance on the far left and far right isn't going to be the same in the distance in the book's gutter. But I want the turquoise lines to allow the page contents to look as "balanced" as possible. I've done a book like this before, and I use the turquoise lines as guides for my abutting the photographs and text boxes on the page. I suppose I could abut them against the pink lines but I'd rather not have things spread out so far.

I hope I am not confusing you. What I would like is for someone to tell me how to set up those turquoise lines, i.e. what numbers to type in above the ruler, in the X and Y boxes. I will use these pages as my master sheets. I've already done one draft of the book and I can see that it's all wrong. I tried to change the turquoise lines myself and they still don't seem right to me.

Is there an easy way to do this? I confess, I am not good with math or rulers or numbers. Thank you for your help!