the numbers are supposed to be on the outside. Slyfo's helm is backwards. so are a few of his other pieces..
one tip is to always make sure the NUMBERS face OUT.
i have a 27 and a 26.5 slyfo. they both fit fine. i really cant tell which one is the 26.5 and 27 if i hadn't marked them.
just think of it this way. bigger is better

(so long as its not WAY bigger then your head

) you need about 2-3in on inside of the helm anyways.
the scale is from the bottom of the chin to the TOP of MC's dome.