You seem to have the right idea.
An encapsulation mount supports a document in three ways:
1. Pressure from having the two sheets' convex sides facing together. All clear film has curvature. Make sure your two sheets have their convex sides facing together, like this )( , and not like this () , in order to create gentle overall pressure in the mount. The sheets lay flatter, too.
2. The tape strips holding the clear film sheets together near the paper's edges create a pinching action. If the tape strips are more than 1/4" away from the edges, the piching action is reduced. If the tape strips are closer than 1/8", there is danger that the document could contact the adhesive.
3. Static charge from the plastic. This increases & decreases with environmental changes, but it is usually a factor in holding the paper in place.
When you place the double-sided tape strips at least 1/8" away from the item and at no more than 1/4" away, there will be a see-through perimeter within the mat opening, and the strips can be covered by the mat.
Be sure to burnish the tape strips thoroughly to activate the bond. To avoid scratching the clear film, use a piece of release paper or a scrap of film under the burnisher.
Some parchment and paper items, especially if thick, heavy, and creased, might have more texture than an encapsulation mount can accommodate. The mount will have a flattening effect, but it may not be enough to overpower heavy wrinkles or sharp creases.
Shameless plug: Take my class, "Clear Film Mounting with Melinex/Mylar-D", which is offered at PPFA chapter meetings and other venues.