In a 2-tier stack I recommend an inside span 1/16" less than the
width of the pneumatic, and the completed pneumatics should be 1/16"
less than fully open when at rest.
For example, if a pneumatic is 15/16" wide (random sample; widths may
vary a bit), the inside span should be set at 7/8" when fully open,
and 13/16" when pneumatics are at rest following regulation of the
assembled stack. Although some are opposed to creasing the cloth,
I gently roll it between my fingers and squeeze to create a slight
crease (before covering the hinge end) so that the opposing folds do
not touch at any point.
For 3 and 4-tier stacks, I set the span at 1/8" greater than it will
be when the pneumatic in its normal rest position. This seems to work
best in all instruments with more than 2 tiers. In practice,
manufacturers varied. Aeolian was particularly generous with their
cloth up until about the time they began building the Duo-Art. It was
probably thought that an abundance of cloth would lessen wear, when in
fact, the formulation of the covering material is far more important.
Using the right amount of cloth allows smooth acceleration; with too
much cloth, the initial burst of suction flexes the cloth, causing the
motion to start out with a "jerk"*.
*Needless to say, I don't think much of the Ampico B method of note
compensation, and consider it a poor substitute for precision in the
covering of pneumatics and in regulation of the valves and piano