I have the Fiskars rotary and it automatically measures the mats for me! I don't have to measure anything.
Its the rotorary and every one that has tried it has purchased one! I had a very expensive guillitine that I bought it an art store and it never worked right, although I could do a lot of paper at once, I couldn't do fine precision work and if the paper was too close to the top where the blad started or too close to the bottom it just tore it.
The Fiskars rotary has a clear oval where the plastic thing goes doen. I lay my picture on the cardstock I'm using for mat and push it up from the bottom of the bed or it. When the top of the picture is straight with the bottom of the little oval opening, it is the perfect place to cut for the mat.
Then I turn the cardstock sideways tapping the picture and cardstock on the table to get the picture straight on the bottom.
Lay it back on the bed with the top of the picture lining up with the bottom of the opening.
It takes about 4 seconds to make a perfect mat with no measuring.
If I want a thinner mat I push the top of the picture 1/4 way or 1/2 way into the opening and the mat is perfect.
I see people a crops struggling to get their mats right nad they just look at me and say, "How do you do a mat in seconds with no measuring or guessing? The Fiskars rotary does it!
Blades are super easy to change - I have two of them.