You got it Tippy.

JR gyros like the 500t are preset to the center pulse width but for various reasons thats not always perfect, thats why you need to subtrim the drift out of them so that the transmitted center pulse matches what the gyro is looking for.

The 401 memorizes center, so yes, if you need to crank the trim over in normal mode and reset the gyro (power off or switch flipping) it finds the new value as center.

The best way really is to trim the tail linkage to get it as close as you can with the servo arm at 90 degrees and no trim, that way the gyro is operating in its most effective range. Remember that with the servo at center you are actually getting negative expo, that is more throw at center than the extremes due to the round servo arm being translated to linear motion. (on Futaba radios this is the same as adding positive expo to a control to make it more sensitive around center).

To farther this discussion and dispell a myth I keep hearing. Expo in the radio will not effect the gyro performance or make it mushy as some believe. Expo in the radio is only applying expo to what it sends out as a stick command, so if you move the stick half way w/o expo its sending a pulse that says move half way. If you crank a bunch of expo in and move the stick half way, the radio is performing a calculation based on how much expo you asked for and tells the rudder to move 1/4 of the way at 1/2 stick. The gyro simply interprets it as 1/4 stick movement, it doesnt know or care how far the stick actually moved.

So some people are lazy they just let heading hold do its thing even if the servo is 15 or 20 degrees off center to hold a hover but they arent doing themselves any favors.

How much difference this makes in the air may be the subject of some debate but I'm a bit retentive about how I set my helis up and those I build for other people.