Changing normal gateTime in mscx

I'd like normal notes to play at 90% length or so for a particular piece without changing the notes themselves for a non-legato effect, and judging by other posts, gateTime is the way to do it.
I tried editing the mscx file , adding 90 in the tab, but it doesn't do anything..
and both work, though.

What's the right way of editing this value?