Hi,
you can mix it as loose as you feel comfortable with. The on-label use is for first coats/bedding; IOW the first coat into which the paper tape is embedded. So it needs enough water content so that it will bond, and not get all the water content sucked out by the drywall paper or tape. Also, on a hot day more water is better so you can work longer runs. Too dry means the tape will not bond and you will have blisters under the tape, and that means removal. The only reason to use it thicker is for pre-tape gap-filling.
Too runny, and it will come back and bite you by causing the tape to bulge.
Second coats I prefer easysand, and for the top coat (and point-up) regular green-lid mud, thinned down considerably.
Casey

Thanks Casey. I followed your suggestions and I am very happy with the outcome. I used the durabond first with tape on top of the drywall, then easysand to build it up and finally, when needed, the green compound. Looks great.

"Also, on a hot day more water is better so you can work longer runs. "

The amount of water added to setting type compounded has relatively little affect on hardening time.

It hardens by chemical reaction, not drying out.

If you sprinkle the powder in a container of water it will even harden on the bottom.

It would need to dry out VERY quickly to not harden at all.

I just meant that on a hot day the vapor pressure is greater and stuff dries out quicker through normal evaporation. The bit of extra water ameliorates the problem. Lack of moisture in the D90 doesn't stop it from setting, (obviously) but it can prevent the paper from clinging to it, which is the whole purpose of the exercise.
Casey