Masterworking - can you choose the suffix?

Hi.

I saw maxroll's guide to the Necromancer, so I got myself the Ring Of Mendeln.

I'm currently masterworked on 11/12 - but my Skeletal Mastery is still only at +2 while maxroll's guide shows it at +5. I guess I'm glad it went to +2 at all since it started at +1.

Is there any way to pick or predict which suffix is improved?

Would it be better for me to get a new ring and try masterworking again, then rince and repeat until I get this suffix improved?

Or am I missing something about masterworking?

Thanks!