The "Carbide collet" is called a Guide Bushing on Swiss machines.
The tightness of the GB is a "magic feel" you will develop over time. Its dependent on:
how consistent your stock is
what tolerance you're trying to hold
how large the stock is
stock material
probably a couple other factors too.
In general, I set my guide bushings so that you can move the bar back and forth with a firm grip with moderate resistance (meaning it takes about 50% of my strength to slide the stock). For smaller stock, a 1 or 2 finger pinch may be all that is necessary. For larger stock, a firm grip and nearly full muscle strength may be needed.
Things to consider:
If you are using non ground stock, you need to mic the bar and find the high spot, then set the tension on a high spot. Otherwise you can jam up or pretzel the stock between the collet and bushing. If you are using ground stock or your stock is very consistent, you can set it tighter.
How closely does the GB diameter match your stock diameter? your stock should never be larger than your GB nominal bore, and no more than -.001 ideally. (I have gotten away with -.002 to -.005 in extreme cases where I just had to make something work, but its far from ideal to compress a bushing more than .001"). accuracy decreases, runout increases as your stock goes undersized compared to the bushing.
What tolerance are you trying to hold? if you're trying to hold +0 -.0003", your bushing will need to be tighter than if you have ±.001. Same goes for ±.005.
How big is your stock? I made a lot of parts from .060 stock and would pinch the stock between my thumb and pointer finger. If I couldn't slide it with just a 2 finger grab, it was too tight and would pretzel. In general, stock below .156" requires more attention to tightness. once you're at .250 and above, the stock wont really pretzel, but there are other considerations like...
What's your material? certian materials like to Gall on the leading edge of the bushing. Once it starts, it basically never stops until it galls too much and your Z1 servo's out. At that point you have to back up the spindle and hack saw off the galled stock. Stainless steels and Tool steel likes to gall, among others. If you have a galling material, set it slightly looser or break the leading edge of the carbide pad with a diamond tip on a dremel.
That should get you started. Basically I would say shoot for 'moderate resistance'. Though that is subjective, to me that means for a medium size stock (.250 - 1.0") you shouldnt be able to move it with a 2 finger pinch. Also, you should be able to slide it with a 5 finger grip with about 25-50% of your maximum strength. In some cases, it had to be so tight it required the spindle to force it through, though that was uncommon.
Hope that helps.