STP distance. Is it critical ?

This necessarily depends on (i) the stylus-to-mounting holes dimension of the cartridge and (ii) the length of the headshell slots.

As to whether any particular cartridge will be able to track the arc, the surest way to know is to try. It only takes a couple of seconds.

Of course if you don't have the cartridge yet you may be in for some guesswork. Stylus-to-mounting hole dimension is not commonly quoted so you'd have to ask owners to measure for you, then compare with cartridges on hand that you know can track the arc.

I am trying to learn about this since I need to mount a tonearm. Altough the template is good, I suspect I could be off a millimeter or two when done drilling the hole and mounting it.

Reading Doug's post, would it be correct to state that if the headshell slots are at least 2 mm long, then this would compensate for a 1-2mm error in pivot to spindle mounting distance?

This might help:

http://www.audioasylum.com/audio/vinyl/messages/77287.html

You can see in numbers what difference there is.

I think the effective length of the arm will change if it is mounted somewhere other than its designed location. I might be wrong, but it makes sense that the arm would swing through a slightly different arc. The manufacturer's template would not line up quite right.