Installing cartr.:slide it all the way to front?

Your arm or turntable manual should tell what the overhang should be. This will hold you over until you get an alignment device of some kind. Overhang should be measured form pivot to stylus tip.

Wow, how can you call yourself Overhang and not know about overhang? Seems kinda odd, no?

Hi Overhang,

The overhang for the Rega 600 is 15mm. That is, your Shelter should be mounted on the Rega where stylus would extend 15mm past the center the spindle on line with the pivot (center mount). Unless the mount of your Rega is loosened, she might not travel that far.
Another way to go at it is the effective length. This is the distance between the pivot and the stylus. It should be 237mm.
But this is just the beginning ... dialing in a cart is part craft and part art. Sometimes dispensing with tools and trusting your ears can bring benefits, as well. So roll up your sleeves, do some research, download some protractors buy a Shure stylus force gauge and lift up your heart for all will come right!

If you position a cartridge arbitrarily you will increase tracing distortion and promote uneven vinyl and stylus wear.

The goal is to have your stylus retrace the path of the cutting stylus across the record while maintaining stylus and cantilever tangency with the grooves at all points. (Perfect retracing and tangency cannot be achieved with a pivoting tonearm - cutting styli move across the record on a straight line radius, not on an arc - but you still need to get as close as possible.)

This is what alignment protractors are for. It is very difficult to mount a cartridge properly without one. Which protractors you can use depend on your RB600's mounting distance.

1) What is the distance (in mm) from your arm's pivot point to the center of your platter spindle?

2) Is this distance adjustable?

With that info we can recommend specific, compatible protractors. Without it we're guessing.

P.S. Mario b's description of overhang was the correct one.

But if the cartridge can't be moved inside the shell, or it's a SPU type of cartridge, how can one get both the pivot-to-spindle distance and overhang right? Sometimes you have to move the whole tonearm, but that would be a violation to the rule that for each arm there is one optimal pivot-to-spindle distance.