One. The stems of capitals are usually bolder than the stems of the lower case. How much thicker will yours be?
Two. Your curves will be wider than your straight lines. How much wider? Lets take a closer look.
Capitals usually have bolder stems than lower case letters. How much bolder should they be? It depends. The best way of deciding is to test a few possibilities.
In Monotype Baskerville, the stems of capitals are 36% bolder than lower case stems.
A curve needs to be thicker than a straight line. At the widest point, the curve of a Baskerville letter D is 11.5% wider than the stem.
When you have worked out the thickness stems and curves for both capitals and lower case, I suggest you make five rectangles.
Make the one rectangle the width of a lower case curve, and another the width of a stem. Make the third the width of a capital stem, and the fourth the width of capital curve. The fifth should be the width of a thin stem, as in the letters A and U. Rotate it as necessary.
Keep them in an unused character slot. To make sure a stem is the right width, copy a template, and paste it into the character slot youre working in.