ml 430 serpentine belt tension
 

is there a good shade tree way to know if its tight enough? like pushing down with my thumb on the top of the belt it should deflect XX mm with 5 lbs (or XX lbs) of pressure? mine seems a little loose.

You could combine these questions all into the same post.

By "feel", no. But here are indicators of a problem:
Squealing belt first thing when started, squealing belt with AC running, sqealing belt when turning hard, especially at low speeds or stationary, squealing belt after driving through a water puddle, squealing belt when revving engine, occasionally having indications of low charging rate by the alternator (warning lights or dimming lights).
If the belt isn't squealing it should be relatively "happy".

Gilly

serpentine belts make me nervous since so many things can go wrong if its not right. so, whats with the 'automatic belt tensioner' i see described in my owners manual on p. 251? do i just loosen it a little and spin the motor over a time or two and re-tighten it? thanks gilley. 'you da man'

No, it's pretty similar to what most other manufacturers use these days, they went ot a spring-loaded tensioner. They may refer to it as "automatic", but it's just spring-loaded. Sounds better than "Spring-loaded belt tightener just like Gillys brother-in-laws 89 Chevy K1500 pickup".

Gilly