Nah, I'd do just what you said...shim from underneath. You might need to make sure the top is held in place securely around the sag so that you only shim the low spot.
I've never checked my home built RT top. Now I have something to do tomorrow that doesn't cost money.... Jim.
Home of Irish Setter Rescue of North Texas
When Irish Eyes are smiling, they're usually up to something!!
At a minimum, I'm Pentatoxic...but most likely, I'm a Pentaholic. There seems to be no known cure. Pentatonix, winners of The Sing Off, season 3