I've already stated how Mathcad is much better for documenting mathematical calculations than other programs. However, the author of a Mathcad worksheet can still make a mess of things by writing a program that is difficult to read. Here are my suggestions for writing Mathcad worksheets that are easy to understand.
Don't crowd the page!
Some Mathcad users try to fill all available space in the worksheet. For me, this tends to overwhelm and I don't know what to read first.
Use vertical layout predominately
Mathcad computes and your readers comprehend the program in a sequential manner so your layout should reflect that manner. Mathcad computes in a vertical, top to bottom manner, but it will also compute from left to right at a given vertical level. For most input and equations, I use a vertical layout as shown below.
The variables are described on the right. You may prefer to put the description on the left of the variable. Be consistent with your choice.
When horizontal layout makes sense
The above cases are shown below.
1: value for variable
The variable is defined, described, and evaluated in one row per variable. The values on the right help in checking for errors.
2: input in one set of units and display value in another
I won't show an example, but it would look similar to above with the input variable on the left and the new units on the right. This structure is useful when variable values are known in field units, but you would like to show the default (usually SI) value also.
3: common properties
The example below shows molecular weight and thermodynamic properties for the compounds used in a worksheet. All of these vectors have the same number of elements (one for each compound) and the compound order (given on the left) is shared. Thus, the reader can easily follow the content.
Other examples might be for physical properties of a gas and liquid. The layout would mimic a table, with rows of properties (e.g. density, heat capacity) and columns for gas and liquid. An extra "column" would be the property name.
The tips I gave earlier about fonts in reports applies to Mathcad worksheets also. You can set up templates for one or more publications, including the math symbol fonts and heading fonts, in Mathcad 15. Prime 3 does not include heading styles.
As I create the worksheet, I keep both the concept of a computer program and a report or textbook in mind. In addition to making the result look better, I think the report concept also influences the grouping and logical structure of the worksheet.